I did find a picture of the suppository tool. It looks like the old style child's top, a flat disk with a hollow wooden dowel through the center. I assume it also had a plunger to push the suppository through the hollow dowel once the device was inserted in the "foundament". I can only guess, but my guess is that the disk was attached to keep someone from inserting the device too far up the "foundament". Suppositories were herb mixtures coated in greases and fats so they would slide easily, and one would hope that the device was also lubricated before insertion. :-)
I am only guessing again, but perhaps the bladder also had a wooden disk attached, similar to the picture in the Voynich, to keep one from inserting the tip of the bladder too far.