How to build triangular walls

I don't think there is a way to build the wall you ask of. I have had similar problems with sims slipping though walls.  May I suggest. 1, remove the bath. 2. remove roof and  the side walls completely and replace the roof. This sort of wall is build when a roof is put in place.  3, relocate the bath to minimize the risk of your sim doing a slip though again. (This seams to happen mostly when your sim undresses/dresses). I hope this works for you, it has for me in the past. If this happens again here are a couple of tricks you may like to try.  (a ), if you have a teleportation pad on the lot, use your sims phone and teleport home. This will send your sim close to the letterbox, this also works if your sim slips through the side of a swimming pool. (b ), place a door behind your sim.  As your sim is already home I'm not sure if the resetsim cheat will work, maybe someone with more knowledge than I can advise you on the cheat.