After seeing Reible's and nuhobby's posts and pics about their DIY Sandfrees, I ordered a 6" Shopsmith sanding drum (555948). That was around January. It arrived in June. I'd forgotten all about it by then. I had some old formica closet organizer parts that we'd just pulled out of a closet to renovate it, so I looked them over and found that they were quite flat/straight/square. So I made a sort of torsion box with 2 of them and some scrap 2x4 pieces, and used one as a tall fence for edge-sanding. It only took a couple of hours, including the dust-collection attachment that connects to the shopvac/HEPA filter (via the Thien separator).
So it sits on the 500 main table with 2x4 pieces holding it forward/back and side-to-side. There doesn't seem to be any need to clamp it on, since it's weight and any pressure you apply to the board hold it down. It's quite stable.
The dust collection works extremely well, since there are so few paths in to dilute the flow. And best of all, the only cost was for the sanding drum.

I thought the formica panels might warp, but it's been a couple of months and it's still nice and square.
Thanks to the SFlee guys for a great idea that works, and to Reible & nuhobby for a great idea that saves a small fortune.
-w4f
PS: I don't see the attachment paperclip icon in the New Thread user interface, so I'll reply to this 1st post and include the pictures there.