I should have stated that you never want to place the light directly above the plastic. Either have the light over the section of lid Not covered with plastic or have the light several inches above the lid. I prefer the Flukers 5.5 inch clamp lamp with a dimmer switch. The dimmer switch allows you to easily tweak the temp to what you need it to be.
Also as an alternative to plastic you can get peices of plexiglass with a few holes drilled in it. It's the same concept just much less likely to melt.