It took me a while to pot up some seeds, but I have now started some Cherry Tomatoes, Sweet Peppers, Black Beauty Eggplant, Long Thin Eggplant and Banana Peppers. Might get some others done later tonight...
My original idea was to start the seeds in 3-inch net cups and put them in a 10X20 tray, on top of a heating mat. But, my 10X20 tray had holes in it, so I could not bottom water the net cups. My next idea was to use the lid of my
Hefty Hi-Rise 72qt Storage bins.
These are the bins that make excellent "mini greenhouses" if you turn them upside down and use the lid as the tray for the pots. The lids are pretty heavy duty, so you can fill them up with pots and lock the clear plastic (bottom) dome on the "tray" and carry them around outside. They stack one on top of the other so you don't waste any space when you bring the plants into the house for the night.
Anyway, I put one of these Hefty Hi-Rise lids on top of my Ferry Morris heating pad and have been checking the temperature of the soil in the pots for the past 2 days. The room temperature in the second bathroom is sitting at 63F. The pots, in the Hefty lid, on top of the heating mat, only got up to 65F.

I think the problem is that the Hefty lid is just too thick for the heating mat to penetrate up into the potting soil. I shot the temp of the heating mat itself with my
Temperature Gun and the surface temp of the heating mat was 85F. So, I suspect that the mat is working OK.
I ended up digging through my garage until I found another 10X20 tray, and that one did not have any holes in it. I am transferring my potted seed cups over to the 10X20 tray this evening and will place it on the heating mat. No doubt, the 10X20 tray is nowhere near as thick, or strong, as the Hefty Hi-Rise lids so maybe the heating mat will have more of a chance of heating the soil in the cups.
From what I understand, you only use the heating mat until the seeds germinate and shoot those first leaves through the soil. Then you don't use the heating mat for them anymore. At that point, I'll transfer the net cups back onto the Hefty Hi-Rise lids for the rest of their time until they get transplanted outside.
In case anyone wonders, I can get (18) 3-inch net cups in a 10X20 tray, but the Hefty Hi-Rise lid holds (35) 3-inch net cups. That was another reason why I wanted to use the Hefty HI-Rise lids on the heating mat. Oh well, I hope the heating mat will work with the thinner 10X20 plastic trays and I'll just transfer the cups later.

Also, since I am now back to using the 10X20 tray with limited capacity, I reconsidered my idea of only planting one seed per 3-inch net cup. I am now putting 3 seeds in each cup and will either thin them out to 1 plant per cup, or maybe break up the 3 seedlings and plant them in individual cups when they get a little bigger. I would appreciate any thoughts as to the best way to handle that. Thanks.