i'll second stephenb's suggestion. I modded my 8" dob recently to allow prime focus DSLR imaging simply by replacing the collimation/locking screws and using longer (and stiffer) springs.
For my GSO/Bintel dob the collimation screws use a 6mm thread, so I purchased six 50mm long 6mm screws, and three new compression springs from my local Bunnings. 10 minutes to fit, less than $20 spent, couldn't be easier
DSLR now comes to focus with about 10mm of focus travel. For visual use I now simply use a 35mm extension tube with my eyepieces.