I made this adapter to hook up dust collection to the odd-size fitting (2″) on my oscillating sander. Start with a hardwood block that is (in my case) is 3″x4″x1-1⁄4” thick. I required a 2″ hole, so I used a 2″ hole saw to drill in the middle of the block.
The next thing is to drill the holes for the split-block-clamping and block-attachment holes. I drilled a 3⁄16” clearance hole, 1-5⁄8” deep. Drill the hole just more than halfway deep – that way, when you split the block, you’ll have a location for drilling the rest of the way through with a 3⁄32” drill bit for the pilot for the screw.
Now drill holes on the face side to attach the 3⁄4” plywood piece; locate them so they don’t intersect the clamping holes.
Now you’re ready to split the block (I used my band saw). After sawing it in half, I touched up the faces on my belt sander to remove just a little bit of material so that it would clamp solidly on the 2″ fitting.
With the block split, drill a 3⁄32” pilot the rest of the way through for the 3″ clamping screws.
Drill a 2″ or 2-1⁄2” hole in the plywood, then install the adapter for the hose and screw the split-clamping block to the plywood piece, with elongated holes on one piece to allow it to open and close.