Rwhite- i am by know means an expert on wet dry but just putting pvc over the side of the tank sounds a little dangerous to me. If any thing were to clog the intake tube, or break the siphon, or just slow down the flow, your return pump would overfill the tank and burn itself out. Also, you would have to find a way to match flow rates of the return pump, and siphon tube, or the tank or sump would overfill. Any type of debris would change the flow somewhere. It might work, but it sounds like too much of a risk to me. I would just do more frequent water changes until you can save up the extra $60 for the over the side skimmer box. Look on the web you can probably find it cheaper somewhere. In fact, water change schedule might be the reason for your declining water quality anyways.
Is the back of your tank actually tempered? I have seen tanks with tempered bottoms, but not sides. if the back is not tempered you could have it drilled on the back. Still not cheap but safer IMO than using a siphon.
Second, just from the shear physical space needed for a sump to operate safely, I think 20 gallons would be the smallest I would use in any system. In my 55g I lose about 1 gallon a day in the summer, about 2 in the winter due to evaporation. The water level would be dropping in your sump since the tank water level would be set by the overflow. So, if the sump is say 2/3 full for safety, that would be a 7 or 14 gallon capacity respectively. 1 gallon a day loss is a large percentage drop in either, but especially the 10 gallon.
If I wanted a wet/dry I would search the web for DIY wet/dry systems that others have used successfully and copy theirs. I always beleive in profitting from other peoples experiences(mistakes) rather than my own. Good Luck!
