Alright, after a couple days of planning, measuring, and building, my wet/dry box is now fully functional. I had good luck on my side that after the box was in, the siphon was flowing and the pump was pumping, I was able to fit the UV
sterilizer and protein skimmer under there like a jigsaw puzzle - barely. The whole thing turned out pretty well and is actually pretty quiet. I patched a minor crack in the box that had a very slow drip, drilled the siphon relief holes which appear to work fine - and all is well.
It took a while to get the inlet valve coordinated perfectly with the outlet pump, but the water level in the box is now pretty consistent. After setup I realized that I should have had 2 valves at the end of the inlet pipe so that 1 can always be tuned to the pump, and 1 can be used as a shut-off valve (without having to use your adjusted valve as a shut-off).
The last problem that is bothering me is that if I lose power for even a few minutes, my siphon will break and the pumps will stop. Fine. But when the power returns, the pump will run until the filter box is dry and the siphon won't be pumping - and I'll likely come home to find a burned-up filter pump. This is a problem because the electrical service in my area is poor to say the least. It'll be a cool, clear autumn day, not a cloud in the sky - suddenly the power will go out for 20 minutes.
So my latest question - Can I get a power strip or some kind of adapter that must be reset upon receiving power from the source again? This way when the power returns, the pump will not start until I turn the pump back on again.
I'll feel much better about the wet/dry when I put this last failsafe in place. Pics coming soon.