Here is my variable suction solution. I use PVC end caps which fit inside the Festool Y adapter. I have three caps each with a different sized hole. The larger the hole in the cap, the lower the suction is at the tool. Here are a couple of photos from a drywall repair/paint job. The hand sanding block works perfectly with a 1 3/8" sized hole in the cap. There is just enough suction to remove the sanding dust but not so much suction that the block sticks to the wall.

