[altterrain]

How much do you want to spend? Twin wall polycarb is the state of the art but they can be made from just about anything. I made mine from 2x4s and some old patio storm doors I saved from a remodel. You can use twin wall polypropylene (coroplast), old window, heavy clear plastic, etc. You just need to be able to vent it so it doesn't get too warm on sunny days.

I used these for my small greenhouse - http://growerssupply.com/farm/supplies/prod1;gs1_greenhouse_covering_panels-gs1_greenhouse_polycarbs;pg103140.html

Lots of other goodies - http://www.charleysgreenhouse.com/

-Brian

 

[altterrain]

Automatic vent openers can help depending on how fancy you want to get.

http://www.charleysgreenhouse.com/3515-Univent-Automatic-Vent-Opener.htm

I use 3 of these (only $20 each) in my little, lean to style, greenhouse, 1 low in the middle and 2 up high on the ends -
http://www.charleysgreenhouse.com/3236-Thermatic-Vent-.htm

-Brian

 

[altterrain]

Usually cold frames sit on top of a garden bed in a sunny spot. Its to warm up the soil 2 - 3 weeks earlier than nature would and protect young plants from frost. What you use to build it is up to you. Mine is a simple 2x4 frame with scrap lumber sides. Don't use pressure treated lumber for anything edible. Insulation may help a bit but is probably unnecessary. The three most important things to keep in mind for constructing a cold frame is 1) angle you windows towards the south to catch the most sun, 2) allow for venting and watering, and 3) build your cold frame tall enough for your plants.

-Brian