If you are looking for cheap but durable, go with cinderblocks. If price isn't as big a deal, go with BRICK! Bricks are NATURALLY occuring clay that has had sand added and baked. There are NO chemicals of any kind that can leach out, that wouldn't already be present in your soil! As far as making sure that the blocks don't move, the easiest thing would be to fill the cinderblocks with your growing medium, and yes by all means PLANT in them! Why would you want to give up good growing space?! If you are going with brick you could always mortar them together! They would then hold themselves together in the shape you desire, and you wouldn't have to worry about any shifting! With the cinderblocks, if you bury them about 2 inches deep, they should hold their place pretty well. I wouldn't expect that they would shift very much. As far as your current clay soil goes, I wouldn't remove it! Clay has a wealth of minerals readily available for your plants. The only issue is that it retains water TOO well, and has a tendancy to get very hard, IF/WHEN it finally dries out. In my own case, (I have a very clay soil as well) I have added a large amount of organic material to the soil. I would dig up (not out) the clay to loosen it, then add any organic material you can find (leaves, grass clippings, bags of humus, peat moss, etc.) and mix it in the bottom of the raised beds. Next I would add the clay you mentioned you have on the side of your house with compost that you seem to have access to frequently, and maybe throw a little vermiculite and peat moss as well. This will give your beds pretty nice drainage, while also having water retaining capabilities. Your clay soil has its downside, but it is also a great blessing! Pure clay is not great for growing, but a clay/humus mix is almost ideal! Btw, by adding the organics (pre-broken down) to the bottom layer of your soil, you will be welcoming worms to come up into your raised beds. (You can't get anything better for your soil than worms!) Good luck, and I hope this helps. Remember, clay is not a BAD thing, it just needs to be managed properly!

I'd put the bed outside the drip line, if there is one. Leave enough room between the bed and the wall so you can easily rake debris from there and work on that side of the bed. A 5- 6-foot wide planted strip is about as wide as you can easily work on without having to climb in with the plants. You could have a 6x40 strip of veggies with working paths all around. Don't worry too much about having to improve the dirt. This picture shows the first year of growing anything in unimproved North Phoenix desert dirt on this side of my house. The key is that there is a drip irrigation system (2 rows of drip tubing running between 3 rows of plants) under a thick layer of rough compost and shredded tree prunings. I scattered liberal amounts of soil sulfur and a small amount of ammonium phosphate onto the dirt at the start. The sulfur unlocks some of the iron and lowers the pH a bit, and the phosphate was a "just in case". Then I dug a small hole just big enough for each transplant, filled it with water and let it drain, transplanted, back filled with local dirt, gently tamped the dirt down and soaked it again. Then I covered the dirt with a 2-4 inch layer of compost and covered that with a thick layer of shredded tree bark. This picture shows what you can do with just the watering system and the coarse mulch, no added fertilizer or compost at all. If you have that much room, dedicate some space to composting things ... http://www.associatedcontent.com/article/1447458/building_a_cheap_wire_compost_bin.html explains my no-turn bin method. If you fill a bin, run a drip line to it and use to grow tomatoes for a season, then break it down and spread it. This one holds the too-coarse stuff from several bins I broke down this spring, with some cherry tomato plants.

@daninthedirt, 8" is just the good stuff above the hardware cloth. Under that is decomposed granite AKA mostly sand with a little dirt. Hubs did dig down pretty far and tossed the soil around, but he didn't amend it. Nothing was growing there before, so it should have some nutrients.. The big worry is gophers. We do our best to control them, but it's an endless battle. There is no rock underneath. Decomposed Granite. The gophers have an easy life digging through sandish stuff. We do have lots of huge boulders on this property, but in this particular spot there are none. I am a huge fan of root pruning pots cuz I've seen the results with and without them. Air pruning a root is like pruning a branch. New branches appear. Same with roots. Nothing circles, just all the roots use every bit of dirt they can. It's a beautiful thing to see. If a gopher feels the need to nibble under the hardware cloth, the roots will just put out feeders. Pretty sure it will work. I'll be dosing with fertilizer often.

I've done quite a bit of winter gardening here on the high plains of Colorado. We get more sunshine than Iowa but probably very similar day length. The main thing really is soil temperature to be concerned with. I wouldn't bother with the the thermometer as I found it made me worry too much. Cold season plants can really take substantial amount of cold. Make sure to vent on sunny days above ~45F. Have your hoops up for a couple weeks before putting in any transplants or seeds to get the soil somewhat warm. The edges of the tunnels will be where the soil is coldest. On the north side of your tunnels have some sort of insulation. Bagged fall leaves in black bags combined with loose straw worked best for me. Inside your tunnels when it's below ~10F at night use a 2nd layer of row covers. The cold season is by far the best time of year to grow the tastiest greens. Good luck.