Lessons/observations from mine, and thoughts on what you said above:
- I lined mine with wax paper to protect the clamping surfaces from glue. It worked, but it was a bit of a pain, one more thing to wrestle with while setting the jig, when you're concentrating on centering the neck down the center and then tightening all those bolts before the glue starts to set.
- Next time I will get an electric screwdriver and hook it up with a deep hex socket, to speed the initial tightening. Final torquing will still be with wrenches.
- I put teflon tape on the threads, in the area between the plates, to try and make it easy to release any glue. It worked about 75%.
- Both bolts and allthread are expensive in larger sizes.
- I used allthread and cut it into lengths. This is cheaper than bolts. It is also much more work to cut and end-chamfer the rods.
- Using allthread (or a bolt with an extra nut at the head side) gives you the ability to pre-set the effective thread length, so that when you do the clamping nut run-down, there is a minimum of thread to run it down on.
- I like your idea of using some rubber to line the cauls - probably silicone rubber would the best at shedding all glues. You might make it so that it the rubber starts at one edge of the working area of the bed, against the bolts, then across the bed, then has a few extra inches to run up the bolts on the other side to protect them. Then do the same thing on the top plate. If you did this you'd want to cut away the extra rubber between the bolts, both so that you have visual and manual access to the workpiece, and so that if you're using any conventional water-based glues, there is air circulation.
- As you said, you may need to open up the holes some to allow clamping a tapered stack. I'd start with the holes narrow, and widen them by degrees just enough to work.
- If you're clamping a tapered stack, you'll need to either pin/dowel it well, or find some other way to prevent layers squirting out. This can develop some impressive clamping force.
"Art without engineering is dreaming; engineering without art is calculating."