The first thing I would ask myself is if it fails what will happen? Is the bad yoke on the tractor side or attachment side? If the yoke breaks you are suddenly going to have a driveshaft flailing around. If it's connected to the tractor it'll either break your tractor or the driveshaft long before you can push the clutch in. If it's on the attachment side hopefully the slip clutch or sheer pin will give before doing too much damage. But the loose end of the sliding part of the shaft could get launched before it stops spinning. I'm all for cheap fixes but not when it could be very costly down the road.
You can try to wedge a feeler gauge in between the cap and yoke. But I wouldn't go very far before checking to make sure it's not working it's way out.
