Flectabed is good stuff, the reflective material is stitched into a thin 'duvet' so it won't rustle and move about like the reviews of the one on amazon said! I have one for my arthritic cat and both of them like to sit on it to keep warm.
If your buns are outside though I'm not sure it's really worth it - because it only keeps the body parts directly in contact with the bed warm...so it would be a bit like you having a lovely cosy duvet but only being able to sleep on top of it. It doesn't warm up the air like a snugglesafe does, so even if the bunny bottoms and toes are kept warmer, the rest of their body will still be cold because the air around the rest of them is still cold. I guess you could try putting it on the bottom and then putting loads of straw on top, but I'm not convinced it would be worth doing as they'd probably just sit on the straw and not be in much direct contact with it anyway.