Playing around with a kind of crazy idea. Our in ground pool has been a problem for awhile now, with a mystery leak in the plumbing, an incompetent job replacing the liner ( where I really should have gone after the jerks and made them do it right, but there are things you don't have the energy for in the midst of a divorce) and now a frozen/locked up pump. All could be repaired but money for the luxury really isn't there and the added electric bill when it is running is substantial.
It is a 33,000 gallon "lazy L" vinyl lined pool, nine feet at the deepest point.
So, I am thinking we can drain it and put a cover over it and presto, walipini. Would leave the liner in place and work container plantings. Should be entirely possible to return it to swimming pool use as long as the liner doesn't get torn up. Which reminds me, there was the mall matter of a good side pine tree being felled into the pool. Dunno what that did to the bottom of the liner. Draining it just to get it cleaned out and see what condition things are in is not a bad thought.
Anyway, I am thinking an in ground pool is a really easy thing to convert into a walipini.
I'd bet a penny that when you drain that pool you find at least one hole in that liner from the tree branches. I agree with your thoughts on Instant Walipini. I'd use PVC Pipe or cheep electrical conduit to form the roof/ door entryway. Something to protect the liner (if it isn't already ruined by the felled tree), since you seem to think you may put it back into pool service later on. I would think some old or cheep carpet would do that the easiest, perhaps turn it upside down so the knap is to the liner side?