I can think of 3 options, perhaps one of them will help.
1 Drill holes on the back of the case to match the mounting holes of the PSU. make a template using the PSU. If you do this, make sure nothing else is inside the case and that you clean it out good before you mount the mobo, etc. metal shavings are deadly to PC parts.
2 Stick that sucker up with some automotive 3M double stick tape. I have some of that on my body kit on my '04 wrx and if it holds at 170mph, then your PSU should be no issue. just put it all over the top of the PSU, put it in place and maybe set it upside down with a heavy object on it overnight.
3. Find a different spot for the PSU, maybe the hard drive bay? Then it has nowhere to fall, and an L bracket can be used to hold it in place.