Hadn't thought about this so had some fun with Google..
If you are experienced with electrical stuff, perhaps open the enclosure and see if you could drill some holes for (plastic) screws.
Granted this is a power supply and probably packed tight so lots of caution advised.
http://www.musiciansfriend.com/accessories/gator-g-bus-8-us-pedal-board-power-supply
The Gator has this feature built in.
Or perhaps make a little bracket that hugs the PP2 that could be screwed in - avoids the obvious risk in opening a power supply.
http://www.musicworks.co.nz/pedal-boards/ubkt1-pedaltrain-pedal-board-power-supply-mounting-bracket-ubkt1/
Velcro is pretty damn sturdy and suits 90% situations but not my personal preference.
Also depends on how permanent a solution you want - screws = permanent Vs Velcro = removable.