If you are able to fabricate yourself, you can make your own mat the size you want. I made mine 3x6 foot, using cable made for keeping house gutters from freezing, 300 watts and I forget the length, available from Charlies Greenhouse Supply. I cut a foam sheet to size, fastened the cable to it with aluminum tape use for duct work sealing. On top of the cable I laid a sheet of 10 gauge aluminum(available from a salvage yard) sealing the edges to the foam. An adjustable bulb type thermostat also from Charlies, bulb fastened to the plate, completed the job. Has worked perfectly for the last 10 years. Al

I had a single standard flat (something like 10x20) sitting on a Hydrofarm mat that would definitely have accommodated at least 2 mats. The mat was sitting on a piece of 1" thick rigid foam insulation, then the solid bottom carrying flat, then the plant paks.

Nothing melted, shorted, or burned out. I don't think it should be an issue. I did have to put a dome on the flat at night so it would stay warmer - those hydrofarm mats just don't have the oomph for the deep plant paks (mine were about 4.5" deep).

Keep in mind that soil temp will be higher than room temp if you have the seedlings under fluorescents. You can probably get away without a seedling mat for most things if you are using fluorescents. I've been starting seedlings for over 50 years without a heat mat, and only broke down and bought the one I have now about 3 months ago because I was starting some tropical citrus relatives from seed, the recommended soil temp was 80F.

I have always had good germination from tomatoes, peppers, and eggplants (and everything else actually) under lights and without a heat mat. Good meaning pretty near whatever the expected germination rate is for the seed - usually near enough to 100% as to make no difference.