What kind of material is your green house made out of? Does using glass keep it warmer then if you use plastic? And I mean hard plastic, not the rolled stuff.
If you want warm keeping plastic it will be proferrional green house plastic , as we say cell plate. But it is expencive. .
Also glass is expencive, but you can get it without money if you ask it from firm, which change windows to houses. You just cut proper size plates from used.
Glass house must be pretty. You just make your own size and with your frames and you install standard shape glass pane in it.
Also you can try if you get ready to use glass house frame elements from the firm (without glasses.)http://www.barretts-leisure.co.uk/Greenhouse%20mnu.htm