A couple of months ago I purchased a Norbert Filmmaker Kit for around $500, which is basically the same thing you made from PVC. I have several small and medium size cameras that we use, so we tried building one of your units. I have to say, yours is much better.
We have built three so far, and along the way made several modifications for improvement. Just wanted to pass these along to you. I have included a few pics to explain better.
First was to change the mount system. I used a PVC electrical junction box for the base. You can use a 2 inch 1/4-20 thumb screw which fits perfect for mounting the camera. I used the bolt to keep the thumb screw inside the box, with a dab of GOOP to keep the bolt locked in place. I also had an extra 1/4 20 adapter "screw" with a male and female threads on the ends. I used this as a mounting means for a tripod or monopod.
I also did not glue the back end together, but used bolts with wing-nuts to hold it together. This makes it easy for transport. I also used "GOOP" instead of PVC cement for the final assembly. This gives you more time to align the pieces together.
For the top and back handles I used foam pipe insulation which is cheap but gives you a better grip. The insulation MUST be covered with Hockey tape. Without it, the insulation will squeak and make lots of noise. I also used 3M spray glue to keep the insulation in place so as not to move. By the way, hockey tape can be found on eBay really cheap. A big roll for around $4.00 which includes shipping.
I also used hot shoe adapters to make an additional accessories mount for additional lights and such. If you round off the corners on the bottom, they will plug right into the top of the 1/2 hole of the coupler. Use epoxy to lock them in.
In addition, I put a couple of "eye" bolts on the corners, to allow the use of a strap, so that if you carry the unit for a while, its nice to let it hang from around your neck for a while. I also used one of these for a job on a helicopter last weekend. The strap assured me that my camera would not fly out the open door.
Each unit is a different size to accommodate different cameras. The largest is a Panasonic HVX200, which I used for the helicopter job.
I also came up with another way to make the rubber bands to stay on the shockmount better. I use black hair bands with last longer and are stronger. After cutting the groves for the bands, drill a small hold and the base of the grove. The band will now lock into place in the hole and not come out.
Wanted to say thanks for your idea, its saved me a BUNCH of money and its so adaptable.