A mount to attach a DVM (volt meter) at 45 deg to a 25mm tube.
These DVMs are under $4 on dx.

http://www.dx.com/p/dd63-diy-0-28-3-digit-green-light-led-digital-voltmeter-black-287339#.UzZPkqJ7Yj8

I used poster tape and screws to attach the DVM.
I replaced tht wires and ran them through the hole on the mount for robustness. Polymorph tidied it up a bit.
It is tempting to waterproof it but I won't do so until I see how hot it gets - it may require air flow for cooling.
I only want to measure the supply so I only need two wires.