I have no idea how many of you will be interested in this, so I thought I would just post this as an "FYI".
I have played Rickenbacker basses off and on since the 70's. I used the top of the pickguard right above the neck pickup as a makeshift thumbrest. It worked okay I guess, but I was never really completely satisfied with that. Placing my thumb right on the top of the neck pickup kinda worked too, but I didn't really care for that either. I didn't like putting my hand farther back toward the bridge and using the top of the bridge pickup assembly as a thumbrest. I prefer having my playing fingers positioned on the strings just behind the neck pickup.
I recently purchased a new 2010 blonde (mapleglo) Rickenbacker 4003. I searched for thumbrests, but most were 2.5" long which is too long to fit comfortably on the pickguard.
I then came across a website belonging to a guy in England who makes thumbrests any length you want. I wanted a clear thumbrest with rounded ends, and I wanted to have it no longer than 34.9 millimeters in length. I also didn't want any screw holes in the thumbrest. This would allow me to epoxy the thumbrest directly onto the pickguard between the front of the pickguard and the neck pickup. (Btw, I have epoxied other thumbrests onto Lakland basses and it worked great, but the Laklands have enough room for the standard 2.5" length).
I e-mailed the guy, and asked if he would make me a clear thumbrest per my specs, and he said "no problem". And his price was the same as the standard size thumbrests he has on his website ($22 plus $3.50 shipping). Within a couple of days, he made the thumbrest for me shown in the pic below. It is 34.7 millimeters long. This will work perfectly for what my needs are. If you play a Rickenbacker bass and seek a thumbrest like this, I highly recommend this guy. I have no affiliation with him. I'm just a bass player who came across his website.
Here is a link to his website:
http://www.classicguitarboxes.com/thumb_rests.php
And here is a pic of what he made for me...