With each different model woofer, the box is tuned differently, appropriate for the motor's characteristics. If you use the Alpha 12, the box will need to be at least 5 cubic feet. So the cabinet size of the tower two would work pretty well, but the cabinet should be tuned a little lower. PiAlign recommends 5 cubic feet, which should be considered a minimum volume for that particular woofer. Other motors will have their own unique requirements for motor chamber, so it is best to match the woofer with an appropriate box. But it's pretty easy to do, and I think you'll be very pleased with your results.

The system you've described sounds nice, and if you need any of the parts, we have all of them in stock. We have everything available from the midwoofers and compression tweeters to the horn flares and crossovers. You might find it easiest to purchase a three π kit in the "π Speaker Kits" section rather than to order parts individually.