ASC makes half round and quarter round bass traps. You may want to look into them. I just bought quarter rounds for my rear corners because, like you, I don't have room for a full bass trap.
Bass trap in a difficult corner
I have a corner that has very little space for a bass trap. On that wall is a large book end which I think makes for a decent diffuser but there is still a corner right next to the bookshelf that is completely exposed. On the adjacent corner is a trap. Will that be enough?