here's how to do it cheap, I am not really into bondage but wanted to try locking plastic pants with a bondage scenario
get this software (it's free but limited to 24 hours
http://www.timelock.rules.it/
what it does is allows you to take a picture and then encrypt it for a limited amount of time, you can choose exatcly or choose a time between this time and that time and a lot of toer features, it even supports some additional safety things, like giving someone else a code that can unlock it in an emergency
so what you do is get a padlock with a settable code, I got a luggage lock with a 3 digit code, you choose a random code without looking, take a picture of it, lock it and spin the numbers, then timelocl the picture with the above software and you are done
for the locking plastic pants i used one of the cheap 3 for $7 priva pants on amazon and some chain from lowe's, folded over the top around the chain, sewed it shut, AN EASIER method would be to get some athletic shorts or swimsuit with the cord that runs around the waist and replace it with chain then lock it
either way you want to do trial runs without locking yourself in to make sure it's not too constraining