Need More info on the use!
Can you give us a few more details on the circuit you are trying to protect? Does it need to dissipate any heat from transformers on transistors or capacitors or any other parts?
Also are there any I/O lines (wires) going in or out of the circuit board. And most importantly, how big is the board?
There are reasonably cheap 'off-the-shelf' type kits you can buy from local electronics shops, but they tend to be low quality type potting compounds. If you are trying to protect this circuit from heat and cold, you may have problems with expansion and shrinkage with potting compound surrounding the board, especially if it is a large board.
Most companies only use potting compounds to protect the circuit from tampering (High secrurity risk circuits) or from the cold, to protect from heat the cooling of the board tends to be done with airflow.
You will also need to be certain the components you are using will be able to cope with the low (& High) temperatures that you want this to operate in.
If you give me some more info on what it is youwant to do eactly, I can point you in a better direction!
Hope this helps,
Paul