I haven't read the other posts, but what I would recommend is to keep the door closed. If you are going to keep the kitten take it to the vet, have it tested for internal parasites (including Giardia). You will probably need to specify you want the fecal to be tested for Giardia since most practices will only send out a fecal float. Giardia is more common in dogs but since you found the kitten outside it is always a good precaution.
My bedroom door is always open. So my cats are allowed in there at their leisure to see my gliders (however they are normally sleeping during the day.) Noel, my youngest cat of 6 will go into my bedroom and sleep beneath the glider cages. she has gotten her nose bitten by one of the boys. When I have the gliders out, the cats are locked out of my room, or whichever room I am in with them, just in case.
I have had gliders get out of the cage and run around the house. LUCKILY nothing has happened but I am hoping my luck hasn't run out the next time they decide to escape.
But if you leave the room and remind yourself to close the door, as you do closing the lid to the toilet when done, it wont take long before the repetition settles in to a natural instinct.