I encountered a similar problem as yours at one point during my more extreme clamping days. I always preferred doing manual squeezing exercises over using an actual clamp. More direct control, you know.
Have you tried just using both hands?
I must say though, even when I couldn’t reach all the way around, I could still do effective exercises. I wouldn’t recommend working with a lesser erection, they were always especially effective for me with a stronger erection.
Have you not been able to reach fatigue?
2005 - BPEL 6.0"x MSEG 4.5" (BEG 4.75"/HEG 4.625")
2010 - BPEL 8.0"x MSEG 5.75" (BEG 6.5"/HEG 5.875")
Goal - BPEL 9.0"x MSEG 6.5" (BEG 6.5"/HEG 6.75")