I've made all my own heater straps - I can keep 3 ep's "strapped" at a time, plus a 2 litre "Bulla" flip top icecream container became a heated ep storage box with some alfoil insulation and 4 (insulated) resistors in the bottom, I also have permanently mounted (but not permanently powered) front lens and viewing end heaters on my finderscope. All of these are powered by the same 12v battery that runs my cooling fan, Argo Navis, etc, via a 5 way power distribution box.
A little bit of simple wiring and a few choice resistors, etc - all up cost for the lot was maybe $20-25 including RCA plugs and sockets, black plastic box, resistors, heat shrink tube AND the 2 litres of yummy ice cream!!!
(Will attach images when I get a chance)