Bottom heat is amazingly effective. Heat matts seem overpriced to me but may be worth it. You can improvise your own source of bottom heat, perhaps using Christmas tree lights. Just be careful that you don't create a fire hazard,

Jim

For years I set my seed starting pots on top of the fridge. Everything germinated within a reasonable time frame, but it wasn't until I tried a seed starting mat (I think I paid $20) that I saw really fast results. This year the tomato seeds were up within 4 days.

Christmas lights work. I made a germination chamber by putting an outdoor strand of lights in the bottom of a plastic tote and set the seed tray inside the tote on a milk crate (to keep the seed tray off the lights). I put the tote lid on loosely. I also had the clear dome on the seed tray to retain moisture. The soil temperature went from 64 degrees to a perfect 77 degrees. The kale popped up in less than 48 hours.

I built a dedicated germination chamber that holds 36 flats on 6 shelves. The lights on below the shelf warm the flat above. The temperature increases incrementally on the higher shelves. The back and sides are covered with foil-covered bubble wrap. The front is three thick clear vinyl panels with magnetic tape to hold them in place. It is 28" deep, 78" wide, and 7' high, with room on the top to store empty flats and trays. There are 2' and 4' shop lights on timers over every shelf, adjusted by a pulley system on each end, so they can be angled if necessary. The flats sit in solid trays so I can bottom water without getting the lights wet. There is a thermostatically controlled fan at the top and a self-closing flap at the bottom to provide ventilation when the temp gets over 78 degrees. I run it for about 60 days in the spring, and about 40 days in the fall. I estimate it took about 40 hours to build and cost less than $300, I start about 5000 plants in it every year.