PVC is too flexible. I made mine out of commercial one-inch aluminium tube stock (google will find somewhere conveniently near to you) and these fittings. Those are in the UK but I suspect there'll be somewhere local for you to get something similar. You could use the steel advertised alongside those clamps, and it might be more rigid, but it will also be heavier and go rusty.
I replaced the allen screws with commercially available handwheels similar to these. They're a standard M8 size and now they're tool-free.
Mine is 8x8 based on the standard lengths of aluminium stock, and each side breaks down into two four-foot lengths using straight couplers. As such, the same parts can also be used to construct a four-by-four frame as well, though I haven't made any panels that size.
I used white ripstop nylon - it's essentially equivalent to what the film industry calls gridcloth. The only downside is that it may not behave in an exactly equivalent manner to other diffusion materials, but it does the job.
The panels are held in using bungee toggles like these. They're not perfectly ideal, being a bit long by default - you need to put a lot of tension on the fabric to keep it flat, especially with things like blue and green screens. Right now you have to wrap the bungee once around the tube to put more tension on it. I should really go through and shorten all the bungees by retying the knots and removing the excess.
The largest amount of actual construction was for the hardware to clamp the frame to the two stands. I bolted a piece of angle to the top of a junior pin to create an upright surface, then bolted a piece of three-quarter inch box section to that using a clamping lever (sometimes called a Kipp handle), with the nut captive inside the box. A claw clamp either end of the box allows the frame to be clamped and held at any angle (and they're good for clamping other tubular objects, too.)
About the only thing you're likely to want to send out is the sewing. I had polypropylene webbing sewn into the edges, to create a tough base on which to mount the D-rings through which the bungees thread. This arrangement is probably better than most of the commercial ones, which seem easily torn by applying too much tension on a windy day.
I got a cheap tripod bag from eBay to put it all in. I remember having to look for quite a big one to accommodate four-foot lengths of metal.
Assume all muslin (and all fabrics per se) will be flammable; spray them with fire retardant.
I can take photos if it helps.