I just repiped my DC with 6" PVC, and one issue I had was that there didn't seem to be any easy way to convert my Unisaw from a 4" outlet to a 6" outlet. I took the plate off, but found a 4" x 19" (?) ramp about 4" off the ground, but even building a transition there was going to mean a big ugly pipe under my TS right extension. The first pic is my solution. It is a large box, even with the base of the saw, and you can see how it butts up against the ramp with the opening in the 3rd pic. There is sort of a shelf built into the box--the dust travels under the shelf, which is a 4" tall channel, which then opens out into another area that accommodates a 6" blast gate. The shelf is accessed by lifting up a cover attached with piano hinge. I did put a galvanized ramp on the inlet side, top and bottom. Its sealed around the edges as well, just because I was paranoid.
It would have been nice to do something like the drawers I see under other TS extensions, but if you notice, the access door for the motor is under the wing and swings out. That means if I want access to allow the door to open, I lose about 50% of the linear space under the saw. I decided, therefore, to go with chest-type storage, figuring I could put lesser used items in there and pile more useful stuff on top.
I had to cut a bunch of stuff w/o any DC on my saw while this was going on, and it built up quite the ugly pile on the ramp and inside the saw. I was a bit worried that without sealing the mating between the saw and the box that I wouldn't have enough suction to pull stuff through, but I turned on the DC and then inspected via the access door and it was clean as a whistle. Not bad for scrap ply and a few hours on the weekend.