I built both my 2pi towers and 4pi's from 3/4" oak plywood. On the 4's I built something resembling shelf braces from 1x2" stock, but on the towers I just jammed a couple of 1x2's firmly in place front to back and side to side. Wayne includes the cardboard tube for the ports with the kit, you just cut them to length.

The link below is a shot of the fugly horns, don't say I didn't warn you. I built the 4's to the plans and just set the horns on top of them. If I was going to go with a hybrid from the start I'd leave the tweeters out of the bass bin and set them on top of the midhorns. The crossovers are just laying on the floor next to the speakers, I removed the 4pi crossovers and will keep them external so that I can use the speakers as two ways or three ways.

I plan on building a new set of horns with an enclosure similar to the one shown on the plans, but as you can see I haven't even finished the 4's yet so it may be a while before I get around to it.