If you don't want to buy the tool, and the gaffer tape / duct tape method really won't do it, try this -
1. Grease the back of your watch with silicon grease / petroleum jelly. Not too thick, but be thorough and work it into the 'grooves' that the proper tool grips.
2. Get some car body filler - Isopon / Plastic Padding / whatever they have in your country. Mix up a ball that's just big enough to cover the back of your watch, and keep mixing it until it begins to stiffen - this will insure that it doesn't ooze or run onto the lugs / crown etc.
3. Leave to harden for the required amount of time and then pop it off, then clean up the the grease from the back of your watch.
4. Voila - your own case back tool. :woot: