ZebraFive: Mine are 4" PVC with a 1/2" hole drilled in the bottom for any water drainage requirements. They are 24" long without the end cap installed and are attached to the tractor using 2 each, 5/16" bolts. I currently have a 5/16" chain, 10' long in each and it takes up ~ 40% of the space. I just did a space-test using 14' of 3/8" chain and it took about 60% of space. My original goal was to make them about 32" deep, but my arms will not comfortably reach to the bottom of that and reduced the length to 24".
These are also handy for carrying long handles tools such as shovels that do not have the D-handle, which is larger than the PVC pipe. I think that 6" PVC may be required for the D-handle tools but this diameter would be too large for my mounting options. I can place both chains into one side and use the other side for a bunch of tools if need be.