I've already built a variant of your slanted surrounds for my son & daughter in law's system. ( the L & R are 1PI's, and the center is a 3/4 cu ft 1PI to fit a space constraint. ). My idea was similar to yours, but the boxes were slightly larger at 3/4 cu ft. I built them with the tweeters below the woofers to take advantage of the ceiling, and provide the dome tweeters with as much free space as possible. BTW, I did build vented cabinets. They simply sound better with lower distortion.
The Alpha 8" is an extremely flexible driver. I call it an "elastic" driver, because it can be used in a lot of different boxes, and sound good in all of them. ( I think Wayne's 1PI is a great speaker design. ) Please tell us about your final results.