This is what I came upon last year... It works like a charm and is cost effective.
Talked to a friend of mine who has been running a small greenhouse operation for 25+ years... and this is what she recommended that I use.... yep, heated gutter cables. Apparently, she has been using them for years.
They come in several lengths, are safe, inexpensive, and easy to use.
Last year, I just placed the cables on the table
squished the gutter clips in between the boards to kinda pin the cable down
popped some wooden stakes onto the tables to keep the trays off of the cables ...
and it worked a charm :)
the cable kept moving around
was a bit of a fuss to deal with
though the bottom heat it provided was perfect.
I screwed the boards into the two 4' tables
went side to side this time, instead of top to bottom
Less stakes were used and no more movement all over the place...
I stretched the cables taut between the stakes
pinned them down into place
so they stayed nice and tight.
Placed the trays of cuttings and seeds onto the stakes above the heating cables...
Covered them with clear plastic to retain humidity and moisture
I often place the trays right into the bags and then keep them pinned closed.
This here is the super high tech garden product that I used to cover them with ;)
This year, I also found this other lovely idea that you might consider... :)