We can't help you design the box with a cut sheet if we don't know what we're working with. The last wedge box doesn't really help, it was probably prefab and sealed. If you make a big wedge box that goes all the way across the back wall then it may work, but again we either need a pic with dimensions or just dimensions. Ther former being better.
That amp won't put out enough to power those 2 se's fully. Maybe try to save your parents a little cash and just run one sub. Don't underestimate them in single numbers. It's a pretty beefy sub.