Here is the way I modified my lower tong to fit inside the receiver.
I bought a 5/8" x 1' copper rod from Mcmaster-Carr (#8966K221)
Tip thread size is 8x1.25mm
The top cut is .230 deep and .735 long.
The bottom cut is .150 deep X 5.500 long.
Then cut it to length to match the upper tong.
This allows full access to the inside of the receiver, even with both rails in place.