Disclaimer: Defeating any safety item is unwise in my opinion. Any clamping of the slip joint sleeve may result in its not collapsing properly in a crash. Use your own judgment as to the risks, also I advise strongly against clamping the sleeve any more than necessary to take the slop out.
Exploded image
The split clamp is a commercial part, steel, with a 1" ID. Also known as a split collar. Should be available from any bearing supply house, but they may have to order it. Make sure they understand you DO NOT have access to the shaft ends, instead the collar must split in two.
A 5/16ths fine thread (24 TPI) hole is then added to each half and matching set screws obtained. The threaded holes should be at noon and six, and perpendicular to the center of the imaginary shaft, but it's not critical that this alignment be perfect.
Images of the installed clamp
It was easy to install the split clamp. In fact, one clamping screw can be installed to hold the two halves together before even getting under the dash. The set screws use a smaller allen wrench than the clamping screws, so I used the force of the clamping screws to deform the outer sleeve of the slip joint. To not over-clamp and risk non-collapse of the joint in a crash, I fully tightened the clamps, then brought the set screws to the point of contacting the sleeve. Next I loosened the clamp screws and advanced the set screws only a quarter turn. Then I re-tightened the clamp screws. This was enough to eliminate the slop on the test vehicle.
Since this was so easy to make and install, I might suggest installing two or three in a row, to compress the sleeve more uniformly. I think this would better preserve the collapsibility of the shaft, and tend to prevent re-occurrence of the slop. Do not install any clamp on the outer sleeve beyond the point where the inner sleeve extends.