Thanks for the info. I picked up a used DC21 and this is great because I have 67mm lens from Tetra housings. I completed the project in one try pretty much on the kitchen table. Just wanted to add that instead of the template I just took all 6 screws out and removed the plastic face plate from the housing. Placed the stepping ring (after sawing) on the table and put face plate on top and marked holes on ring. All a bit tedious but a great DIY modification that's easy on the wallet for a change.
I'm glad it worked for you. I was reluctant to remove all 6 screws as I didn't want to break the seal on the port, but testing that it's still waterproof without the camera in it can easily get around this concern.
If anyone is going to use this modification with just the internal flash (like me), then by moving the diffuser to the end of the close-up lenses, the shadow of the lenses are almost, if not completely reduced. I did mine by using a piece of elastic (from a flight luggage label) and drilling another hole in the diffuser. The next two images show what I mean. Plus here is a shot of the technique being used.