I finally got the collar jig finished for the Rigid. I was able to use the equipment at work, so this isn't an easy jig to make just due to that, but I figured I'd share it anyway and maybe someone else can use this idea to make their own with what they have available.
I started out with a 1.5" pipe nipple as it seemed to be the best fit on the 1.5" drum. I had to remove the weld from the inside (where they make the pipe) to get it to clear the sanding drum. I used our lathe to take care of that and then used the lathe to turn down the collar to as small as I felt comfortable with (so that my offset would be minimal).
Once I had that, I made a mounting plate for the collar and pressed the collar into this to the right depth so the thin area would just stick above my base.
I used a piece of plywood for my base, a couple of holes tapped in the mounting plate and counter-sunk in the plywood, and it was all mounted up and ready.
Here's the collar and plate before painting:
Here's the collar and jig mounted to the plywood base:
Here's the collar in place with the 1.5" sanding drum:
Here's how my offset worked out. Shouldn't be an issue to take this into account:
I'm going to go back and shave the collar down to around 1/8" tall (its almost 1/4" now). Turns out the oscillations were taking the sander down past the top of my 2" thick piece and not giving me as smooth as a finish as I would like. This also showed just how different the sanding drum surface was along its length, so I'm also going to look into making an aluminum sanding drum to use with this. I'll have to go to stick on sand paper though.