Use pressure treated wood. Retreat cut edges with something like Copper Green. Cut right triangles to the appropriate angle to create a level platform for the box to sit. At this point either glue and screw to the box, or attach to the window sill. I'd probably attach it to the box myself, to aviod potential water intrusion to the house through the holes in the sill. As an alternative, instead of buying a window box, just build one that matches the slope. As a second alternative, make a few small concrete forms. Tack it to the sill and pour permanent pads (to replace the PT right triangles). You will probably want to dowel it into the sill if you go that route. PJ

Adding assorted filler stuff to the bottom of a container is suggested only if your container is very large and you don't want to fill the entire volume with potting medium. I think it would be advisable for you to fill your window boxes with your potting mix, foregoing that layer of bark in the bottom. Your plants will need all of the root zone space they can get. Screen is a good idea. If you find that, over the course of a few weeks or months, that they become clogged with the fines from your potting soil, you may want to try something coarser next time. I've learned to use the plastic grid used for needle point.

In general, plants with thick waxy leaves and those with fuzzy leaves are more tolerant of dry conditions as well as those with silver leaves as DtD suggested. Along the lines of dealing with the drying out factor, you might also want to suggest that your mom add a product to the potting mix that is often referred to as water crystals. They are made of a polymer that when dry looks and feels like rock salt, but when water is added, absorbs much more (maybe 40 times) their size in water so that they look and feel like gelatin chunks. (It only takes a very small amount to do the trick, and before starting use of these, I killed many, many pots of plants.) The plants grow their roots to these little reservoirs which slowly release the absorbed water. It means that I only have to water once every day or two for my south and west exposure pots, rather than 2 times a day. Don't use little bitty pots, since they will dry out faster, and glazed or plastic pots will dry out more slowly than unglazed terra cotta or those coir lined wire baskets. Your mom's window boxes, unless lots deeper than standard, will probably only be useable for succulents since they just don't have enough soil to prevent drying out. I also have grown thyme, lavendar, bay, and rosemary in pots and they have done well, though only the thyme and lavendar will survive the winter here, and the lavendar only if I tuck it out of the sun and wind so the snow will bury the pot (which I leave on its side for the winter so that it doesn't get too wet during a time when the plant isn't using water.) Your mom, being farther south and in an urban environment, may be able to keep all these except the bay plant alive outside. I have grown pots of mixed succulents, both hardy ones, like some of the lower growing sedums or hens and chicks (Sempervivums) and annual or tender perennials like jade plants, echiveria, portulaca, many of which do just fine inside for the winter. Licorice Plant (Helichrysum) is a silver (or chartreuse or variegated) stiffly trailing plant that does well in pots. Many of the flowering annuals like trailing petunias, million bells, lobelia, etc. won't do well in settings where they may dry out, so unless you get some other suggestions, she may have to stick mostly with plants with interesting foliage. Old-fashioned geraniums (Pelargoniums, not perennial geraniums) are amazingly drought tolerant, though ivy leaved geraniums aren't. I've grown both flowering geraniums and scented geraniums and wintered them over on sunny window sills and started new plants from cuttings. They may need more water (especially the scented geraniums) than some of the others, but are probably worth trying. As for flowering perennials that she can leave outside all year, I'd suggest she try some southwest natives that like it dry and let the soil dry out between waterings. Some plants to try might include some of the hardier western Penstemons and Agastaches. The general rule of thumb for overwintering perennial plants in pots is that they should be about 2 zones hardier than plants in the ground for that zone, so in zone 7, look for zone 5 plants. Other places you might have luck would be to ask in the container gardening forum on GW or to Google for images of "succulent pots, mixed." There are also books you might borrow from the library, like "Designing with Succulents" or various books on container gardening. One final thought, though I haven't tried this: if your mom ends up with annuals in some pots, she can get bulbs in the fall and plant them after the annuals are done, leave the pots out for the winter, and start bringing them inside in late winter/early spring (maybe February?) to force and have some early spring color.

The balcony on the right is the neighbor's the townhomes are set up as grouped duplexes. We also have an equally large balcony on the back overlooking the pool. A lot of space to fill! These balconies are not really convenient to use because we have sheers in the bedroom windows that we keep closed for privacy, so we have to battle through those, and the balconies are much more exposed so less pleasant to sit on. We have a large patio out from and out back, both of which are much more private and have shade, so we have no shortage of sitting outdoors space. I would have been happy with no balconies and just some railings with window boxes or something! Yes HOAs can be limiting but to be honest I am glad, when you live in close quarters you don't want to have everyone putting up random stuff everywhere or storing junk on the balconies. Also the HOA is responsible for painting and repairing balconies so they want to ensure that plant pots are not sitting in water rotting the deck etc.