40 should not be hard to deal with....
Here is my recommendation....if you can get 2 shop light fixtures and some regular fluorescent bulbs, set them up and give them light at least 12 hrs a day...keep the lights pretty close to the plants..no further than 2"...remember the rule of light, the power of the light diminishes by the square of the distance from the light...reason for saying 2 shop light fixtures is if you allow 4" per plant that will give you 2 rows of 12 per fixture....so 2 fixtures will give you enough room for 48 plants...
If you want to set up a "temporary" growing area, you can use anything to close them in...I use plywood, insulation, and mylar (for reflection) to build my grow boxes... but I am trying to keep the plants between 80-85F. If you are interested in building a grow box for seedlings you can use year after year, there is plenty of information posted in the forums that contains information or you can simplay ask someone and I am sure you will have all kinds of help and advice...