I hope you don't mind if I jump into this discussion. I am the guy who developed the PBase® control plate currently available on the market.
Just to take out any expectations on the outdated links I have seen in this thread: none of them will get you to a site with an actual product available. The old ABM example on Reverb was a single private item that has been out of stock by ABM for a long long time. I did extensive investigation on the internet worldwide for my own P-Bass and decided to develop this control plate myself as I could not find a proper one anywhere.
I'd like to push back the remark on the steel PBase plate being 2 mm as opposed to the 2.5 mm of a plastic pickguard as well: an original Fender Jazz Bass control plate has a a thickness of 1.5 or 2.0 mm depending on the production era, which says enough I guess.
Furthermore, if you want to install the PBase control plate "stand alone" without the remainder of the original pickguard 2.5 mm would be far too beefy! I have developed the PBase plate with stand alone installation in mind (for older P-Basses with an internal pickup --> control cavity routing), so a "straight ended" Jazz Bass type version would be a lot less elegant than my solution, which is inspired by the previous ABM form (see pictures below).
I will definitely consider a "straight" Jazz Bass inspired version as I can clearly see the benefits of this when if want to preserve the remainder of the pickguard (also with a top-routed wiring channel). However, this will be not until I have reached the end of my current production batch. After all this is a private investment. Please respond to this remark what you think of this.
Finally, I will attach some pics of my own P-Bass with the PBase control plate and a few pics of the plate placed on top of an original Fender pickguard.
I hope this helps in "our" discussion and kind regards from the Netherlands,
Peter Niessen, Positech Design
positechdesign.com
P.S.
Please also have a look at my readily available LTwist neck torsion gauge. This measures exactly your longitudinal neck torsion (of your bass/guitar that is

). So far, such a gauge is not available on the market anywhere worldwide, you will be surprised!