I use the Velcro brand Velco (I get it from RS Electronics), specifically the heavy duty stuff and it sticks almost too well - use two much Velcro and it can be a struggle to remove anything. ?

What is the surface like on your pedalboard? The smoother the better and cleaning the area with isopropyl alcohol before you stick the Velcro down helps a lot. Also leave it for a day or two before pulling off a pedal - the glue cures harder over time.

I used a shoe lace and screw it onto the board. I then sew a little strip of velcro on each end and I have a velcro tie down. I strap my power supply to the bottom of my board. Just an idea. Hope it helps... Let me know if anyone wants a photo.

Hi Alan, thanks.

My board is covered in velcro, and I use the sticky velcro on the actual supply. I can't get the sticky stuff to stay on the bottom of the power supply. But I'll give the rubbing alcohol a try, and ask around for that velcro you mentioned. Cheers

I just used normal Velcro from a material shop and stuck it on with contact adhesive and it holds quite well - I don't have a heavy PSU but my crybaby wah stays in place even if I flip the board upside down.

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.