Congrats!. You get a 10% discount on my answer (lol - 10% of not much isnt' a lot of discount...)
Anyway, I would just make either 4" or 6" thick panels and mount them horizontally and straddle that wall/ceiling junction. You can certainly do a chunk style design but you'll have a lot of waste material with the non-45 degree angle.
If you want to buy something premade, we use the 244 in this type of application all the time.
Bryan