There are many materials you can use to cover the tank and yess all of them will help hold in heat but some work much much better then others. I have had alot of experience with this with Brazilian rainbow boa's which need very high humidity.
Best is probably laminate or sealed wood which is custom fit to sit on the screen frame(not the screen) but inside the edges of the tank rim so that it cannot slide and it forms a tighter seal.
Plexiglass or other plastics can be used but are known to sag.
Plastic wrap works but is extremely unatractive normally and is a fire risk if exposed to heat.
Lasty damp towels can work in a pinch but require daily misting to help hold in humidity. This is the least effective method and the most labor intensive.
Hope this helps,
Sean.
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1.1 BRB
0.1 Pacific Gophersnake(unproven Hypo)
1.0 Amel Pacific Gophersnake
0.1 Striped Anery Pacific Gophersnake
1.1 Triple Het TPRS's
0.1 Silver TPRS
1.1 Amel Bloodred Corns
0.1 Abbott Okeetee Corn
0.1 Blizzard Bloodred Corn
1.1 Thayeri Kingsnakes
0.1 Reeve's Turtle
0.2 Amstaff's
1.0 Pudytat