In the detail below, the first line is secured under the end of the boom, then goes up through the eye of the boom end cap to a block attached to the mainsail clew. It then runs back to a swivel block on the eye of the boom end cap, then up toward the camcleat, where it goes internal to a swivel block.

Inside the boom, another line is secured inside the removable end cap, up to the block, as a cascade, back to the exit block shown below left, then up to the camcleat, with foot block beyond.

My Main is short enough from the end of the boom to allow the two blocks. The rivets have been scrapped, with the end cap secured with a through bolt, with reinforcing plates, plus one self tap fixing at the base. This facilitates access to the internal lines, without too much fuss.

It works well - you get a tremendous tension, over a long adjustment length. The whole lot will pull back about two feet - in terms of fitting the mainsail