Buggy "func_physbox"
Hello everyone !
I have a problem when I make physboxes in my map : i can't pick them up when pressing "USE" (E), I can't even push them. 
Under flags have you checked PHYSGUN can always pick up or Enable Motion on Physgun grab?
Another thing; physboxes have a certain mass; it depends on the material and the size; if the mass exceeds 85kg you can't pick it up anymore. I'm pretty sure there's also an option for override mass; maybe that can bring it to a pickupable range too.
He can't push it, so it doesn't have anything to do with mass. Maybe, make sure you flagged, "Enable motion".
The algorithm to calculate mass is glitchy and tends to return huge values. Try hollowing it out with Tools > Make Hollow to a radius of 4 units, or sending it the Mass input.
Thank you for all your anwsers ! I am trying your solutions.
I have specified "metal" as material. Maybe that explains the mass problem. I'll check the flags.
Also, as the physbox consists on a lot of parts, is it possible to export the brushes as a .mdl model ? It would be a good optimisation.
Sure, use Propper. There's a tut somewhere around here to use it with the P2AT.
It seems like the texture you give the physbox also plays a role in the mass calculation. You could try applying one of the wood textures to see if it changes.
However, physboxes I wanted the player to pick up usually had one of the standard test chamber metal textures. They mostly where 32 units long and high and 8 units deep, it could be that 64x64x8 also worked, I can't remember now.
Instead of hollowing this object or anything else, there is a keyvalue in the object called Mass Scale. By setting this to a really low number this will make the object lighter.
e.g. 0.05 (or 5% of the original mass)
News : problem still not solved. I managed to make a 16x16 physbox I can pick up (with a wood texture, wood material) but 32x32 fails until I crave it.
Also, It seems that the "mass scale" variable only affects on the object's falling speed.
My last hope is to turn my physbox to a prop with propper. I'll keep you informed