My cushions are only 22" width with maybe 5 or so inches depth
A couple of years ago I bought some of the cheapest, widest(60"), tough material I could find ... I bought 4-5 yards of some nice "clearance" material, turned the cushions up-side down and placed the cushions on the fabric
Then I pulled everything tight(not super tight) ... all around, and using heavy sewing thread, very randomly sewed all the odd corners and all ... all together, worked fine for two years ... this would be the third year.
I'm thinking about "improving", on this short-cut answer this year, by doing basically the same thing(maybe with new material), and placing grommets every foot or so, then using twine to tie everything off.
I might even add some batting to the top of the cushions first.
The whole thing cost me about 20-25 dollars tops, it holds firm, and even though I have to put up with seeing the "bunching"(although still flattened), at the corners, and NOT seeing the more squared look of a finely sewn, shaped cover, the cheapness of the covering, along with the absolute cheapness of changing the cover on a whim is nice.
Maybe I'll always keep two sets of covers, so I can change them(that is, untie the backs), almost like sheets
If I really get serious, I could get something a little firmer and place this "form", on the bottom to help hold a shape even better.
Not the nicest looking, but doesn't look bad either and 20-30 or so dollars(with grommets), is one heck of a lot cheaper than $2000 ... or even $200.