Thanks for the suggestions, guys.
I ended up going with one of these from Testar
https://www.testar.com.au/collection...ondary-mirrors
Hopefully it should do the job. I've now fixed the issue I had with the fans so I'll have that as well, and I'm making it so I have a 12v astronomy hairdrier in my kit too. Three redundant systems should be enough, I hope!
I'm intending to run the cable along a spider vane and keep it locked in place with a fold of gaffer tape over the top.
The tricky thing is the dew controller. I don't like the idea of it heating things more than necessary, so I'm going to ave a crack at making one of the projects from around the web. Dew heaters seem insanely expensive for what they are. $300 for a couple of pots attached to some mosfets seems excessive when there is no readout or ambient temperature / humidity sensing.
Thanks for the idea about cannibalising the heating pads, Patrick I may try that with some other projects I'm considering. And it's good to know it doesn't affect the diffraction. I'd be curious to see how you attach a dew control system, to the filter slider. I imagine that will be an issue that I will have to solve at some point too so I'd love to see what you're done there.
And thanks John. I know I said I prefer the wrap-around ones. That was before I googled a bit more and realised that the elliptical ones are glued to the back o the secondary itself, duh! A much better solution if you can manage the wires efficiently.
Cheers
Markus