Update
Carrier survived a weekend of offroading. Never came undone, and never added the pin.
Then of course, I come home, get a car wash, and the carrier flips open on the street, shearing off the t-handle lock-open pin, and putting a good dent in the rear corner body. Was it the water that caused it come off, or did I forget to lock the handle in? We'll never know...
I did some more research on these pull action clamps, and it seems the key is to have them pull over center, i.e. the latch plate needs to be in a perfect right angle to the latch. Mine is not, the latch arm forms a ~110 degree angle when clamped, when it should be 90. The bracket on the bumper is 1/4" thick, and this moves the latch plate too far away from the latch.
The only solution I can think of is to weld on a steel tab/plate to the end of the carrier arm to bring the latch dead over center of the latch.
Back to the welder...