Here is the one that I like the most from my short time as a kayak fiddler, or is it fidgiter? I wanted to use my rear scuppers for intake and exhaust for my bait tank. This is just a focus on the exhaust which is also used to control the water level (infinitely adjustable) and empty the tank. I took an ordinary PVC compression fitting for 1/2 inch pipe. Here is one cut in half (normally has a fitting on both ends.
And here it is unscrewed.
Here it is modified and attached to the bottom of the tank (originally the connected end looked just like the bottom in the photo.
The modification was to cut one of the screw on ends in two (careful to leave threads on both end), which made a nut for the bottom of the tank and kept the cap for the seal inside. Here is approximately where I made the cut.
I then screwed the smaller "nut" piece down the threads just far enough to allow the cap to still function when attached. I cut a 1 3/8 inch hole in the bottom of my tank where it would have access to a scupper with the tank installed. The 1 3/8 hole was tight enough that I needed to screw the fitting into the tank, but I also gooped the nut. Now all I had to do was add a piece of PVC pipe through the cap and fitting and tighten the cap. Here it is from the inside in the up/full position.
And here it is in the down/low position.
I ended up cutting the compression fitting in two, to use the other end on my next tank, so here it is now.
A couple of use things I have noticed are that I need to leave the cap fairly loose to allow easy water level adjustment, or tighten it to transport my bait. And simply unscrewing the cap and removing the pipe quickly drains the tank. Also, rubbing the PVC pipe with candle wax helps out.