Hey David,
Teleskop express offer a wide range of FF which would suit your needs, they can be found here -
http://www.teleskop-express.de/shop/...orrectors.html
Also yes collimation is extremely important, but it's never affected my guiding when I dont use correctors. I've gotten 60min sharp subs at the native FL without a FF or good collimation. You do not require any expensive equipment for collimation either (Infact, absolutely nothing except a screw driver and allen keys). I managed to get it pretty much bang on with just those tools and this guide -
http://www.deepskyinstruments.com/do...re_Ver_1.0.pdf
FYI I personally have the Orion field flattener which works beautifully, especially for $149 new. It does only have 55mm backfocus though. I think the WO P-Flat worked quite well (not sure of its back focus). Lastly as per Graham's post, I also have the CCDT67 it works beautifully on both my RC8 & 80ED (And my 120ED when I had it). I know you're interested in imaging at the native FL but just thought I'd put it out there
