Hi Phil,
How's your skills with electronics? It's very easy (and cheap) to make a simple dew heater from nichrome wire, there's a few write ups on the net. I start with a simple single heater which needs no controllers, and now just made myself a 4ch controller and can make the straps to suit anything I want (for your mount, telescope, finderscope - you can customise the shape and power to whatever you like
). This cost me 1/10th of buying the same thing from the shops! If you regularly swap the EPs then you could even build a little heated holder box where you keep your EPs in, so that way they stay warm and when you take them out they won't dew for a while.
I'm happy to build you one if you like, or just give advice if you wanna do your own
send me a PM if you wish
Cheers