If you just take the back plate off you can see the mechanism through the existing holes- no drilling or fabrication needed. The brightly colored master combos are aluminum bodied so they're easier to work with. You should be able to get the back cover off in a few minutes by filing off the crimped ring that holds it on. When you file it off, leave a few sections that you can bend back later to hold the casing on.
Master padlocks are fun as far as they go, but I'd suggest picking up a cheap group 2 lock on ebay to play with. I'd actually recommend this over a better combo padlock since it's so much easier to disassemble a safe lock and see what's going on. They regularly go for under $20, so it's not a major investment.
And I'll stop there since this isn't safecracking101...
