:P DM06...great minds think alike...I bought similar poly siphons in the auto-parts section at Walmart. Each siphon had two valves. Note which way the valve seat runs...use only the valve that opens downward. I carefully cut the valves out of the siphons. Using a dab of silicon adhesive, the valves popped into the scupper holes perfectly. They have stayed in all summer. The scuppers drain more slowly because the valves have a smaller diameter...but at least I don't have to sit in water any longer. Cheap to replace about $2.
Too bad Hobie does not manufacture a screw-in scupper valve.
Don't get me started on Hobe and a lack of valves lol. Sounds like your siphon was a bit different than mine if you were only able to use one of the valves. With mine I just sawed the housing apart and was able to slide out two identical valves.
Really looking forward to testing this mod out, and you mentioned the best part: $2!