The Apogee guide I bought turned out to be unusuable garbage. The laser cutting was imprecise and far too energetic and had burned some of the flanges that fit in the rail so thin they crumbled coming out of the plank. I took some measurements with a mind to having a similar guide laser cut, but I've not followed up on that. I'll probably go the 3D printed route once I get my Prusa up and running.
Edit to add: the Apogee guide was also too loose to work on Apogee's own thin-walled tubing, which is what I'm working with. It might work fine on the thicker-walled tubing Estes are using for the So Long.