Since the cushions have a plywood bottom which will warp if the wood gets wet, I doubt you can totally waterproof the total cushion unless you varnish the plywood to seal the wood. You are better off taking the cushions inside and store them in a convenient dry location so moisture doesn't get into the foam and a mold/mildew farm begins. You can still cover the set with the plastic to keep it clean if you are in the southern or southwestern states and don't get a lot of snow...there are clear vinyl covers to cover the whole set at discount stores, catalog stores, etc. or the big box and discount stores have clear vinyl drop cloths you can use for a cover. In the northern states, we take the sets inside to store them for the winter, or leave the frames outside covered and take the cushions inside and store them in a dry place.

The tightest fit will be if the fabric doesn't stretch and the cover is made a tiny bit smaller then the filling. If you are new to sewing, I suggest you make the cover the exact size as the cushion filling keeping in mind that you can always wrap the cushion filling in ploy-fill to make it fuller. good luck!

This is gorgeous, 2.625 yd Brunschwig Fils Upholstery Fabric Monsoon Floral Coral and I think it would work beautifully, but note the odd sizes of the pieces if you are interested. Brunschwig fabric is very expensive, and this is a 'relative bargain' on eBay.

I may have to get rid of the lounger. It's 30+ years old and I could never replace it for what I paid for it, and I like it, hence I keep hanging on to it. I could replace it with any number of seating options that wouldn't need cushions or would look good cushions-optional, but I mostly try to keep using what I have if I can . . . but I'm downsizing. I do like having it where I have it. Maybe I should just sacrifice the cushions to the gods this summer . . . but I priced replacements, they are not cheap, particularly ones made from good materials. So that's not going to work, I don't have the money to buy cheap ones every year or so. I've ruined a couple of cheap pillows by trying to wash them. I don't have the skills to replace the cushioning material. That would be one option, having ones with stuffing and covers that could be washed. I have a few pillows on our screened in porch that work out great that way, the cushions wash up great, the stuffing doesn't come apart. I can use bleach for the mold. But they don't get constant sun and rain exposure, they are under a roof. Just a little dampness over the summer.