There's something about binoviewers on a giant scope that just must be done. When I recently became the owner of a 20" f/5 classic Obsession much to my surprise, I busted out the binoviewers first thing but was disappointed that I could only use them with a 2.5x powermate, even a 2x barlow or 1.6x OCS didn't work. I ran some numbers (and had a friend run some numbers too) and we came up with a 4" shortening of the tubes. From 70" to 66". But rather than cut them and forever be stuck with using an extension tube when not using binoviewers, I opted to have a whole new set made so I can go back and forth depending on the night. So I called up a local metal company and they were able to cut the 8 tubes of aluminum to 66", 1.25" OD for around $80. Not too bad! I ended up getting .065" wall vs the stock .049" in order to keep the weight balance similar, and it's a good thing I did, as it is a little more prone to taking off when you remove the eyepiece than it was before. The upside of this is it can take a whole bunch more weight at the eyepiece before it ever starts to think about taking a dive.
I used William Optics Binoviewers with the stock 20mm eyepieces that came with them and it gave amazing wide field views of some DSOs and planets. I found that when I wanted to switch to a regular eyepiece, or use the binoviewers with a barlow, that a 2" diagonal gives the perfect amount of backfocus. So, while a little weird looking, it is actually much more ergonomic for viewing the low altitude planets right now with no impact on the sharpness of views.
The only downside to this setup, is the mirror is nearly perfectly sized on these 20" f/5 Obsessions with a 3.1" mirror, so I'm losing a little aperture with this mod. To really take it to the next level I probably should upgrade to a 3.5" mirror.