I too have a CV64/3D & scandoubler; it is a right pain in the a**** to get it in/out...
What I ended up doing was (I think.. so long ago
removing the bus-stop/faceplate on the end of the CV64/3D so that you could put the scandoubler in first, and then the CV64 could stick out the back through it's slot while you are putting it in.
Then somehow I magically got the faceplate back on... that doesn't sound right does it...
hmm.
Cheers,
- Paul