Pool and Spa
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What material is the pipe? what size pipe? how accessible is it (how much room do you ave to work)?
So it's not a pipe it's a chamber? made of what material? It can't be plywood. Maybe the plywood is on top but the chamber has to be something else? PVC? HDPE (polyethylene?)
So if I understand you correctly you have some type of plastic chamber directly below the plywood in contact with the plywood. You retained the piece which I assume was drilled with a hole saw which means the plug/cylinder also has a hole in the middle from the pilot bit. Is the tank material stuck to the back of the plywood or did it fall into the chamber? Is this spot out of the way? not a trip hazard if something sticks above the plywood? how big is the hole?
You didn't answer all my questions. Maybe best to get back to me when you get home. A key question related to the type of fix is: is this out of the way.
I'm thinking your best bet would be an expandable plumbing test plug. A plastic one would be best since a steel one would corrode over time. I asked about where the hole was because this option will stick up above the plywood. The only other option would be to use the piece that was cut out as a plug the use silicone caulk as a sealer and to fill the pilot hole. Silicone was used to hold the glass panels together in the New England Aquarium's 4 story high main display tank. So it can certainly withstand the pressures in this case. Of course this method would require than the piece that was cut out of the tank is on hand.
Do you have any more question?