Tools and supplies required:
1 X Tap handle
1 X 8-32 hand tap (plug or taper)
2 X 8-32 X 1/4" conical tip set screws
1 X VP trigger
1 X Flat 1/2" wide hand file
Step 1: Remove the stock trigger and pin.
Step 2: Tap the trigger pin hole on the right side of the frame for 8-32 threads (go slowly, 1 turn in, 1/2 turn out; and make sure the tap is sharp, the composite the frame is made of flexes a little and may deform if the tap is dull, ruining your threads).
Step 3: File the inside of the frame (right side only) until the trigger fits into the frame.
Step 4: Start the set screws into the threads until they bite, but not protrude into the inside.
Step 5: Position the trigger's bearing hole in between the set screws and clamp the trigger down with the set screws (being conical tip, they'll be self-centering).
If you do it right, it should look something like this and eliminate side to side play:
This might also work with the rails' UL frame.