In my greenhouse I have a "propagating "box = 6ft x 2ft x 6ins lined
with plastic sheet , filled with sand and containing a heating cable
laid 3ins deep.
I stand my seed trays/plant pots of seedlings on the top of the sand -
seems to work quite well.

The question : should I keep the sand damp? If so would it be worth
putting a plastic sheet on the sand in order to retain the dampness of
the sand?