Since the question was coming up in a different thread that was only related to strings I thought it might be a good idea to post some pictures of my modification on (what I think is) a Clevinger keyhole EUB. I got it used, it was converted from a four string to a five string (four string grooves still at the bridge). I didn’t like the pickup solution as I got it from the previous owner, so I changed a lot.
The bridge was screwed to the top because there was no raised saddle and without it the bridge would have fallen towards the fingerboard due to extremely different string angles at the bridge.
I placed some angled wooden posts screwed into the original holes to angle the bridge top lower to half the string angle at the bridge in between post and bridge are two pickups, piezo disks cut into shape and sandwiched into hard wood. I also made a double piezo buffer for the two pickups (not enough room for a HPFpre because of the battery and output jack) instead of the passive BG electronics and removed the post for the endpin to have a straight one (much better for the bow).
Then I replaced the endpin rubber with a big one for a walking stick ore something similar that had a lot of rubber below the shaft and cut it so that the there is full contact with the floor when I hold the instrument. That brings the bass back into the right place if I move a bit.
The rubber foam (originally made for a bicycle) at the body support was there before, but a bit old and I made it a bit better so that it cannot turn around. I cut one spirally to fit the metal rod tightly and fixed the ends with tape and then put the second one over the spirally cut one. That way I have a really good contact with the fake bout and it won’t slip.
There is a second pickup, a German Bassbalsereit (now ANS or AMS, cannot remember currently, like a very thick Wilson K1), that is passive and ends in a balanced XLR connector.
The bridge hole is not tapered as it should be for the pickup, so I inserted some cardboard to better fit it in.
There is also a piece of rubber foam tube around the XLR connector, so I can stick it between body and underlength. It also damps the underlengths that way.
The fingerboard has a high curvature. It seems that a former owner wanted the bowing angle for each string to be like on a four string instrument, so a lot of wood was removed from the sides of the fingerboard. At the end of it there is almost no fingerboard height left. But that was not my idea (bad for pizz), I got it that way. Does not help with my 5-string DB, the bowing angles a smaller there (even though maxed out as much as possible).
But enough talking.
Here are some detail pics:
