You could build it up a lot of different ways. I would use some concrete board and cut strips and set them to the exisiting Curb with thin-set (taping all of it with a wide mesh tape). Wait a few days and then re waterproof the top and inside of the curb. Leave the face (dry side) un waterproofed.
I don't see any re-inforcing fabric in the corners and the one wall does not look waterproofed. This pan is designed to be a topical waterproofing process.