I made a solid hull for a slatted floor dinghy ages ago. Just sheets of ply steamed to fit and a keel/transom from softwood, then the whole thing filled with foam and epoxied over. Attached to the bottom of the temder with straps, and worked quite well. A lot of effort though. With hindsight, I'd swap it for an airdeck tender...
Some manufacturers sell 3-piece floors as an alternative to slatted ones...ask avon.
I made a one-piece plywood one for a really old/tired dinghy : had to use tyre-fitting lube to get it in & never tried removing it, but it worked a treat.