I had lots of trouble trying to get a dial indicator to mount to my Solidoodle carriage and give consistent readings of the bed level. I finally realized I shouldn't even try.
This is a dial indicator mount that attaches directly to the X axis rails on a Solidoodle 2 (and maybe 3 if the rail separation is the same, but the openscad file is included for tweaking if you need to).
I've experimented with this a little, and can't think of any more ways to improve it, so I'm calling it done now.
Print the railcrossing part. Make sure it fits snugly around the back rail and the front rail rests in the obvious position at the front of the railcrossing. You may need to file the back a bit before it fits properly (I didn't have to do much filing though).
Note that the extra holes and crosshatch pattern are there simply to provide strain relief when printing the part (I had lots of trouble with it curling up).
Drill or file the holes for the magnets till you can get a magnet to fit flush with the top of each hole (a small hole is provided in the bottom so you can push the magnet out after each test fitting with a toothpick or stiff wire). The back hole has a hole above it for running a drill bit through from the top.
Epoxy magnets in each hole flush with the surface (the part is designed for the 1/4 inch cylindrical neodymium magnets which seem to be very popular, but the openscad source is available if you want to tweak the size of the magnet holes).
Print the dialclamp part. Drill out the holes where the M3 bolt goes if it doesn't fit, and tap the hole on the other side of the clamp with an M3 tap (or use an M3 bolt with acetone on the threads, but I have a real M3 tap, so I used it). You could also modify the openscad source to put a M3 nut trap on the side if you have an M3 nut available.
Note that when you put the dial indicator in the clamp, you want to tighten the nut just enough so the indicator can't move up, but not tight enough to crunch the plastic.
After the epoxy has set, glue the dialclamp to the end of the railcrossing, and you should be ready to make bed measurements. That's just a theory, I haven't yet reached that point (my epoxy is curing as I write this). Hopefully I'll be able to get consistent results when measuring the same thing more than once. The magnets should hold the railcrossing solidly to the rails, and the dialclamp should hold the dial indicator solidly in position.
More babbling about this can be found at http://tomhorsley.com/hardware/solidoodle/solidoodle-railmount.html.