Cut a 1/4-inch thick piece of wood 3 inches in diameter to use as base for the block that you put the sandpaper on for the drill disc sander. The wood is the support for the extension. Cut a disc of thick heavy rubber 5 or 6 inches in diameter. You can get sanding discs in both sizes, so just choose what you prefer.
Purchase a 6-inch wood bit extension bit. You can find extension bits online at sites like Amazon or at your local hardware store.
Drill a 1/4-inch hole through the center of the wood circle and the rubber disc. Use a 5/8-inch countersink drill bit to create a valley to recess a number 10 or 12 screw head.
Apply pressure to the countersink head screw to push it into the open end hole on the wood bit extension. Use a the appropriate screwdriver to twist it further into the shaft.
Adjust the set screws on the wood extension bit with an Allen wrench until there's no give and the set screws grip the threads of the number 10 or 12 countersink screw. Make sure the head of the number 12 countersink set screw in recessed below the level of the rubber mat.
Use the adhesive on the back of the appropriately sized sanding disc to stick it on the rubber. Insert the shaft of the extension into the drill chuck and tighten down. You now have a drill disc sander.