Run down to a local pharmacy/Tescos and see if they have any body warming pads. I think theyre usually in the sporting sort of section (with Deep Heat and joint support stuff). You just knead them for a bit and they heat up nicely and stay hot for several hours. I cant think of anything else off the top of my head.
the poly box should hold the heat best if not opened surely? id have thought it could mainatin for a couple hours but maybe not... Good luck.
Heat the air around the bator/the room with what you can, can you get hot water without elecy?
Keep the box closed and put a towel over it. If you're likely to be without power for more than a few hours, get some heat packs / a hot water bottle and put them outside the box. This will reduce the thermal gradient and slow down the heat loss. It's important not to open the box though. Damp vermiculite / perlite holds a lot of heat, so the eggs will be fine for a few hours.