A few ways of doing it, you could just make the coldframe unit then just drape the lumisol over the top of it as I did when I first built mine and weight it down. By the way the polythene in this photo is not lumisol
OR look at this link, it shows that I eventually made a lid for my own although I used polycarbonate in this photo but it might give you an idea of how to go about it.
http://www.shelledwarriors.co.uk/for...ng+area&page=4
I havent fitted any lumisol as yet so not got anything to show you but I would think if you build a frame work for your lid then stretch the lumisol over it you will have to screw more wood on top of the lumisol to hold it on place.
The whole design from start to finish of mine is here
http://www.shelledwarriors.co.uk/for...t=warming+area
Hope this gives you a few ideas