The problem with liners like you are looking to build is two things.
One to make it sealed from the rest of the cage would take caulking whcih in turn would make a removable liner very difficult to remove.
I assume you wan a removable liner, if not just make the bottom of the cage out of plastic.
The reason to seal the cage is that I find boas generally deficate where you don't want them to so inevatablly a boa will eventually deficate up the wall and this will run down and seap between the liner and the cage interior.
To over come this you could make the liner taller but then why not make the entire cage out of said liner material.
The other problem I mentioned is getting the removable liner in and out of the cage, assuming again this is ou intention. To fit snug it will require very close tolerances making removal difficult includeing the removal of animal front window and all cage decorations if any.
I think your intention is right but you may be over thinking the solution. Simply making the cage and lining it with a cheap vinyl will IMO be a better solution. But making a cheap replacable floor is another potential solution. No matter what it is made out of it will eventually need replacement.
Although I may have read your entire post incorrectly and if so I apologize but a tub liner IMO is not the best solution due to its complexity and difficulty in use.
Good Luck and don't turn off the idea machine.
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Jeremy
"I am become death, the destroyer of worlds" July 16, 1945 Robert Oppenheimer
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