I spliced this line together in order to hold the boom tight (to the side) so that it doesn't move around in rolly anchorages. This end is a quick release with a Wichard shackle and the other is a simpler climbing spring-loaded shackle.
(2014-02-08 16:15:43 NIKON D7100 with a "18.0-250.0 mm f/3.5-6.3" lens. [f/10.0, 1/640s] ISO 100)

This homemade line pulls the boom over to one side and put it under tension so that it doesn't move back and forth at anchor. The closer end is a quick release mechanism on a stay and the other is a normal shackle.
(2014-02-08 16:15:31 NIKON D7100 with a "18.0-250.0 mm f/3.5-6.3" lens. [f/10.0, 1/500s] ISO 100)

The sheeting system for the main sail on Zanshin doesn't use a traveller but a system of 3 sets of blocks and boom vang to allow the correct angle and downward tension to be achieved rather then use a traveller track on the coach roof. This system works very well under sail, but has a big drawback when at anchor. If one pulls in the sheet tight enough to stop the boom from moving around while rolling around in an anchorage there is a resonance created in the boom track which produces an ungodly loud humming noise and boat vibration. This has been the same on my previous two boats and the solution there was to move the traveller over and change the boom angle.
One wants to immobilize the boom to prevent unnecessary wear and tear on the gooseneck, so I opted to splice a line with attachments at each end and attach one end to the boom and another to an outboard stay and fix the boom in position that way. The system only takes seconds to set up once at anchor and is a simple solution to a rather bothersome problem.