Some people build their π cornerhorns with everything permanently attached to the bass bin. I've built a bunch of 'em that way too. But another way to do it is to pin the midhorn to the bass bin and to the tweeter. You can use a wooden dowel or aluminum rod to make positioning pins. Woodcraft sells table leaf pins that are perfect for this:
I just use the 8mm insert and then have 8mm aluminum pins made to fit into them. That way, the bass bin and midhorn both have holes in them and I install the pins when assembling the system.
You can see the inserts in the bass bin, and the midhorn has them too, aligned so they position just right. I insert the dowels into the bass bin and lower the midhorn onto the pins.
Insert the pins in the bass bin before setting the midhorn on top
If you're going to use the speakers at high power levels, you might want to place a rubber mat between cabinets. The material used for shower liner pans is perfect for this. Cut it to size, and it makes a nice gasket.
Lower the midhorn into position
The midhorn has a pair of holes on the top and on the bottom. The bottom holes pin to the bass bin and the top ones are for the tweeter.
Positioning holes for tweeter on midhorn
Assembly is pretty easy. The face of each is flush and centered, so alignment isn't complicated. Once you get them lined up, they drop right in place. When everything is assembled, it is really solid and looks like this:
For those of us unable to use a tape measure, ME, these Dowel Centers are required, also from Woodcraft Ya drill a hole in one part and put the center in it. Then ya lower the othere part into place and sit onit. The pin on the center marks the drilling location. Muy facil!