Hi Angela,
There are a couple of things going on here that are complicating matters for you.
1. Unfortunately, there is not a ‘cold temperature’ resin. Resins do best in the low 70’s F ambient temperature range. It sounds like you are already doing a lot of things right like using a heat source to get rid of bubbles, but there are a few more things you can try:
*Make sure your casting room is in the low 70’s F
*Warm the bottles of resin in a hot water bath before use
*Gently warm up your resin surface before casting
2. You may be getting bubbles from your wood pieces that escape into the resin. Before pouring the resin into the engraved area, you should try sealing them with something like Cactus Juice.