I made my own propagator with soil warming cable and a thermostat that goes
up to 40 degrees C (in old money that is far greater than 100F.
I used cheap old contiboard, lined with heavy grade plastic sheet. A couple
of buckets fall of sand and a sheet of secondary glazing glass on top. It is
in excess of 3 feet long and just under 2 feet wide and works wonders in
bringing on bedding plants etc.
If I made one again I would make it even bigger and line with polystyrene
insulation on the base and sides to keep it even warmer and at less cost.

AndyP

--
"Wisest are they that know they do not know." Socrates
"If more of us valued food and cheer and song above
hoarded gold, it would be a merrier world" JRR Toliken, The Hobbit