Here is the tool, ready to go. I bought the main compressor from Sears for $25. It wouldn't fit the DSM head, even with the extended tip on it, as the tip hit the edge of the head long before it pressed against the valve retainer. Putting a socket against the tip and valve retainer was a bit better, but the extended tip still interfered with the head. So I put the flat extension on the compressor, and figured out I'd need 2 sockets to get the right extension to avoid interfering with anything.

I had bought a $100 all metric set from Sears, and it came with a small (14mm?) sparkplug socket, and 18mm deep socket that are NEVER used on DSMs. So I used them to save a bit of money. If you are buying new sockets to use, you will save a bit of work by buying the small sparkplug socket (remove the rubber insert), and a deep socket that the hex of the sparkplug socket slips into.

Here's a closeup of the sockets. I welded the 18mm socket to the flat extension that came with the compressor, then ground the hex end of the sparkplug socket until it fit into the end of the 18mm deep socket, then welded them together. I then cut a window out of the sparkplug socket. NOTE that the location of the window is important for ease of use - it faces away from the rest of the compressor, so once you have it on the valve, you have the most room to rotate the compressor around if needed to access the keepers on the valve tip.

Here's a pic of the alignment of the sockets and the "pusher pin" (don't know what else to call it). The pusher pin should be as close to centered in the socket opening as possible, to keep the force of the compressor running straight through the valve stem. I pushed the pusher pin all the way into the end of the socket when welding it to help on the alignment.