I've tried dozens of ways to
lock the vac trash. The only way that has worked for me is to cut a sheet of stainless sheet as wide as the trash
door is tall (about 12"). Weld angle to the ends of the sheet. take the existing locks and hasps off the
door. Wrap the stainless sheet completely around the vac so that the two welded angles meet up and hold the
door closed. Drill 3 hole through the two angles. Put a padlock in one hole to
lock the two angles together. Put nuts and bolts through the other two holes and tighten to pull the sheet tight around the vac. Pop rivet the sheet to the vac so when you clean out the trash and unbolt the sheet it stays in place ready to close back up. Sorry for the long description, a pic would have been better but I don't have one handy.
Without looking close you can't even tell there is a wrap around the vac. The wrap looks like part of the vac canister. It will only work on standard round vacs. The bums that dig through the vac debris typically have something to pry with, but not a couple of wrenches to loosen the wrap.
They have been pried on a couple of times, but never any major damage and never gotten into (except the time that some guy figured out that he would just pry open the top
door, cut the filters and reach down through the cut filter to get his treasure). I've done this to my locations where the vacs wouldn't make a couple hours before someone tries to get into the trash if it is not locked.