Help Sought on Cobra Trailer Lock

I wrote to Spindleberger about this, but they are on vacation for the
entire month of August. Hopefully someone on RAS can provide a
solution before the Spindlebergers return.

I have a 1983 Cobra trailer with a metal top. On the rear of the
trailer, one of the sliding rods in the locking mechanism (the rod is
the one for the right side of the trailer) is no longer attached to
the silver T-handle. It appears from marks on the rod that a set screw
came loose.

I cannot determine how to re-install the rod and re-secure the set
screw, as the lock mechanism itself and the remaining sliding rod are
completely hidden inside a hollow square tube. I don't want to
randomly drill out rivets, as there are quite a few of them and they
are tightly secured!

Can anyone give me precise instructions on how I might proceed to
repair the rear lock mechanism? Thanks!

-John