I think the idea of using a dimmer instead of a thermostat is false economy.
What you should be aiming for is a soil temp. of around 46-50f, with that
you can get away with an air temp of around 40f, or even down to 36f with
some plants.
You would be better using a time switch, but a thermostat is best as with a
hard frost you will want that extra time with the cable running on full to
maintain the temp.
At least with a timer you could give extra heat during the night, esp. from
around midnight to about 7am.
I wouldn't worry about a bublewrap tent, a single layer of polythene to form
a tent away from the glass will serve just as well, and if you have a fan
heater in there to keep the air frost free then you should do well.
Last winter I used a polythene tent inside a 24ftx 40ft tunnel. the tent was
7 ft in the centre and down to 5 ft at the sides. This cut the area being
heated down by around half, and meant I could get away with less powerful
heaters, and a lot less electric.
As spring came on I raised the tent till it was only reducing the air space
by about 15%.
One advantage of this was a great reduction in condensation as there was no
Cold film against the Warm air.
As the sun warmed the house I didn't have an insulated film (as you would
have with bubble wrap) keeping the heat away from the plants.

--
David Hill
Abacus nurseries
www.abacus-nurseries.co.uk