If $39 for 2 Watchamacollar V-rack clamps is too rich for your bank account you can’t get the seatpost clamps pictured above for $4.20 each after discount. They are slightly larger than the tubing on the Big Dummy so you’ll need to wrap the area you are clamping with some tape to get the right fit. A reasonable compromise given the cost savings.
Watchamacollars in action:
Note that the Xtracycle Watchamacollars have a built in o-ring to keep water out of the Big Dummy/Xtracycle sub-frame. So if you use the DIY clamps and live in a wet area you’ll want to use something like some inner tube to keep water out.