Ok, I got my Al spindles today and added a bearing to each so they're ready to install when I get time. The inside diameter of the spindle where I want the bearing was about 15.85mm. So a little sanding with a mini-dremel sanding drum and they pressed right in for a snug fit.
Here's where the bearings are normally:
Here's where the third one is:
Inside view:
The reason I did this is that one of my 15x21 skinny bearings blew out a while back and I wanted to try to give it a little help. I don't know how much this will help, but it was easy enough to do. For the rear carriers, I have an 18mm od, 16mm id Aluminum sleeve that I'll use to install an extra bearing.