Stan,
I put my QRPlabs kit in a sandwich of 1' styrofoam. I first cut three pieces 4" x 5" Then I cut a 2" x 2.5" rectangular hole for one sheet. I used aluminum duct tape on the edges and turned the radiant barrier film to the outside. I made a little grove for the antenna lead to exit and then one of the other pieces on each side and taped the thing together. See picture at
My suggestion is don't make it to complicated. A secondary value of the method that I used is that it is very light tight. Incident light on the LED will change your frequency, perhaps as much as small temperature changes.
I power the transmitter from only 5 volts and have had excellent frequency stability. It very quickly came to temperature of equilibrium. My power out is 220mA and I'm operating at 10139955Hz, from DM62pj New Mexico.
Cash Olsen KD5SSJ
ARRL Technical Specialist
