Ooh, doing elastic is an interesting idea! I'm in the "same boat" as you... the cushions in my 1978 MacGregor seem to be okay as far as the shape they're in, but the upholstery is right out of the 70s - orange, brown and tan plaid - the v-berth cushions look like slices of pepperoni pizza LOL
I've got fabric from the po (incidentally, it's my sister) so I'm gearing up to deal with the cushions. One of my concerns is, in fact, that that current ones do have vinyl bottoms... which seems like a good idea, considering there are bilge access panels underneath every cushion... I'm hesitant about putting just fabric on the undersides because of condensation I've already seen on the vinyl.
It sounds like you're talking about making covers like how fitted sheets are made. Elastic really isn't hard to do at all... when you do that bottom hem, do it wide enough so your elastic will go through it.
Maybe another option for me is to just stitch some vinyl on the bottom where the cushion fits over the access hole.
*sigh* and I was JUST poking around for info on cushions before heading out to get the right size needles for the fabric. Now I have to stop and rethink it again.
I already hit the jackpot in searching for info when I read that I could put Velcro on the cushions instead of a zipper
And I'm still not sure whether I want to put piping along the edges - it seems like it would help them last longer.