Re: Another number Dilemma
Sweet little rig!! Wow, California sure is interesting in how they manage that stuff. I'm in Indiana but have taken my inflatables all over (Not west coast though) and I've never seen anything like the number placard you have. Is it mandatory? Do you have to use those exact pieces? Anyway I've had several boats, both PVC and Hypalon (5 or 6 - I stopped counting) and the registration numbers are always a pain. One thing i have learned, always consider resale value. Right now your thinking you will have that boat forever but next year things may be different and a guy from another state might turn away if he has to deal with covering or removing permanent numbers. Generally on a fiberglass boat permanent really doesn't mean forever but on an inflatable it does. My first boat, a Sea Eagle, was simple. I bought a stencil kit at K-Mart along with a sharpie marker and an hour later numbers were on the boat and it was off to the lake. I used a clear plastic clip-on ID badge to hold the annual renewal decal. Ugly but worked great, however the boat has absolutely no resale value now and I don't use it. In Indiana registration numbers will change with a new owner so nobody wants it.
In your situation I recommend two options, both of which get the numbers OFF of the tubes which I highly recommend for a number of reasons. Regardless if you try to fold up those orange plastic stick-on number plates with the tubes it will never work regardless of what glue you use.
One idea is to create a rigid number plate just below the seat, and above the tube. You could cut out a piece of plywood (Or better yet SeaBoard, or that corrugated plastic used for yard signs) and affix the number decal to it. Then use some brackets to screw it down to the boards holding the seat. I think a stainless or brass door hinge would work great and let the plate sit and flop around on top of the tubes while the boat is moving around. That would be "Permanent." However consider how it will work when you deflate the boat and try to fit everything in your trunk.
Another idea, seemingly easier and less cumbersome, would be to create a number plate (Same as above) and use UV resistant marine rope to hang it off the sides. Ordinarily rope & knots may not be considered permanent, but you could use some aluminum crimps instead of knots. They sell these at Lowes/Home Depot in the hardware section. Externally they look like a figure 8 and you thread the rope through to create a loop, then smash it closed with a large flat screwdriver and a hammer. Very permanent but you will always be able to undo it by cutting the rope off with a utility knife.
When in doubt use diplomacy. Mock-up your idea and go right to the police station and ask them their opinion. Get an officers name so you can always say "So-and-so told me this would be okay." You have an investment there and I'm sure the law enforcement guys will be happy to help you out and tender an opinion.
Incidentally I think MEK would dissolve PVC material, if you want something to really, truly stick very well FOREVER such as a patch use 3M 5200 glue (Google it)