I seem to remember somebody on the U-Bass megathreads posted that Kala sent him a replacement.
One guy I jam with lost his a year ago.
It has no effect on the sound, so he doesn't care.
The magnets were not one of Kala's best ideas.
It's not like you need to be in there all the time, only when you change strings.
The original solidbody S-U-Bs used a plastic cover with magnets, but the California solidbodies use screws now for sure, and maybe the S-U-Bs as well.
If you know anyone who does woodworking for a hobby, they could make you a replacement, then fasten it with counter-sunk screws. Forget the magnets.
A luthier could certainly make one for you.
The world started going to hell around the time they started putting tweeters in bass cabs.
Yorkville-Traynor Club #231