Surely having a water resistant box is a little academic isn't it?
Whatever box you mount it in will loose its water resistance as you need the controller k n o b on the outside to adjust the flow and turn the switch off. If it was on the inside of the box then you would need to unscrew the lid each time.
IMHO the unit would be better mounted down in the box as any water would tend the drip away and not find its way into the electonics board as in Tosh's unit. But it's more difficult to get to and wouldn't get an award for best design. If your van is dry (not dripping water), does it matter?
I wonder how many times a day Tosh actually adjusts his flow rate now his controller is 'part of the furniture'.
As regards the other question from Cleanclear - yes the higher the working amperage the better. The first controller you have would be fine for a low current draw pump such as one used in a backpack. But going for a higher amperage like that starts to put the cost up into the realms of the purpose built units.
They say that the max a larger Shurflo pump will draw is 6.2amps delivering water at 100psi , but that is running the pump 'flat out'. I don't know if the current draw would change if the motor is expected to do the same thing at a slower speed. I would think it would. There must come a time when a stalled motor under load becomes a 'short'.
If it didn't then we would never have a burnt out or cooked motor.
I appreciate that our motors haven't got field coils but use magnets, but the armature or rotor has got windings. If you put too much current through them they will heat up. This changes the resistance of the wire and and causes the motor to draw more current. Maybe the burn on this controller is evidence of that, but maybe it could just be the controller's weakest spot.
Nat Jones always suggested that part of this controller installation is to reduce the pump's max delivery pressure by adjusting the pressure switch. I agree with this advice. Nat has his adjusted at 65 psi. This will reduce the electrical stress of the electronics having to push the cutoff pressure to 100 psi every time.