You can buy a rod handle clamp on-line but you can make it easily yourself for about a third of the price.
Go to your local DIY store and buy a couple of 500mm long 8mm diameter threaded rods with matching washers, nuts, and wing nuts.
Rummage around your garage/shed for an old piece of wood (I used a scrap of three quarter inch thick plywood). I cut two rectangles of wood 3 x 2 (in hindsight 4 x 2 would have been better). On both, draw diagonal lines from the corners so they bisect right in the middle giving you the centre point.
For the bottom plate drill two 9mm holes for the threaded rods half an inch from each side. Drill a hole in the centre point that is large enough for the butt end (widest end) of your blank to easily slide through. Bolt the threaded rods in place onto the bottom plate.
For the top plate, follow the same process but use 10mm holes (so that the top plate easily slips up and down the threaded rods with a minimum of fuss) and use a saw to open up the edge, down to the centre hole in an almost vertical sided V this makes it easy to slip the blank / handle in and out of the clamp. Slip the top plate onto the threaded rods, followed by washers and wing nuts.
Photo of wood plates before assembly:
Final assembly should look something like this:
