I agree with GM. You could lose the horns in the 828 cabs and save a good bit of space.
Don't remember who much room they use up, but it's substantial.

The 828 box with the back of the horn flare open has a net volume of ~11 ft. You could probably get away with a net volume of about 8 feet with the 515B. That means a box of 40x19x22" (3/4" ply). That should save you a lot of room.