I take a 1/4" thick piece of aluminum big enough for a grip warmer to stick to.... stick a grip warmer... add a short wire hooked up to provide low or medium heat and put that in the bag with some good wiring attached to it...it heats the bag well and will not burn the element.
Last one I did, I stuck the heater to the plate... then I put a second same size plate over that with some dots of ultra black (any similar is ok). Will hold up better and makes a great 'radiator'
Yea, just lay it in the bag and it works great...If you are looking to defrost goggles....not sure you'll find a bag that can take that much heat though... but heck, it never hurts to try.
I set up a guy a few years back with a car seat heater in his tunnel bag a few years back...worked well.... sandwiched between some 18 ga aluminum sheet and sealed around the edges with silicone.... ran it on low.
A thought that I've not tried yet... grab some cheap pieces of aluminum computer heat sink material from ebay... cut to size with a chop saw or band saw.... round the corners with a file and use that...would be a better 'radiator' in the bag and probably be more efficient.
There are also some cool flexible silicone 12v heating pads out there.... say 20-30watts.
Plenty of silicone 12v 3D bed heater elements out there... in the 20w range that would work well... DO NOT run anything over 30 watts in my opinion https://www.ebay.com/itm/153315479375