Making a soil warming propergater

I have received my 20 foot warming cable and now waiting for the thermostat to arrive, started on the box to put the cable in but need your help now.
It says put a plastic liner in the bottom of the box first then 1 inch of sand and grit then lay the cable and add 2 inch of fine sand, now the tricky bit.
Can I put on the very top a capillary mate?
The bottom layer of plastic if laid will have no drainage, confused.
The propagator will be 4 feet by 2 feet plus 3 inches high and will sit on my greenhouse staging, I will also add a plastic top later on.

This is and old gardners world with toby who built a propagator
http://www.bbc.co.uk/gardenersworld/video/toby_30minfix/heated_propagator.shtml

Dmg