There are two easy ways to control the mast while adjusting the rigging:

#1:Remove the trap handle from the shock cord on the side you are tightening, and have a partner stand away from the hull a few feet and hang on the trap wire, this will bend the mast a bit which enables an easy adjustment and it cannot fall the other way while being held.

#2:Put the traveler all the way out on the side you wish to adjust, sheet in ALL the way (block to block), then go ahead and make your adjustments. Note how this will only work with the sail up, which is always a bit sketchy. I get nervous sheeting in the main while the boat is on the trailer or sittting on the beach. This methode DOES allow you to do all of this without help, but I would strongly recomend method #1.

As far as I have been able to figure out, the only truly bad thing that can happen in Cat sailing is having the mast come down, so always do whatever you need to make sure that does not happen!