I don't know why your inserts don't work. They must be big inserts or you are trying to put too many in the tray. 8 inserts is what should go in it. That tray is wider than the ones I use. In any case all you need is something to put under the inserts. Take a tape measure and go to the dollar store and look at their plastic cutting boards or raid the recycling and put some plastic lids under them or plant saucers or wash and cut out the sides of a milk bottle. As long as they are plastic and not something that would mold or mildew it should be fine they should work.

It's one large 72-cell tray. Just sags a bit in the middle, but I like your idea of raising it up a bit. Thank you!

I have had the same problem and I do lots of seedlings for a cut flower production. I have found that sometimes it is brand of flat or tray, some are not intermixable. As dowlinggram said, find something to prop up the centres, it is a pain.