The original 38mm av-bay kit includes 2 bulkheads that clamp around a 2" long 38mm coupler tube, which seals the electronics inside. The outside faces of the bulkheads have the ends of the threaded rods exposed, so you can attach your ematch wires. But attaching your harness or something else to the av-bay is left up to the user, and the 2.5" length of the threaded rods (the longest pre-cut Al rods McMaster sells) doesn't leave a lot of room at the end for other attachments. In my own rockets I have been cutting my own longer threaded rods and attaching an additional FG bulkhead to the ends, where I can loop a harness and add a charge holder. 
Shown above is what I like to do for threading the harness through the bulkheads and attaching a BP charge holder. The charge holder is an Al threaded spacer with one end drilled out to the ID of the red ematch plastic covers. A counter-sunk 8-32 flat head screw fits nicely underneath the bulkhead to attach the charge holder and seal the bottom end of it.
I decided to make this solution more accessible for everyone, so I did a production run of harness attachment bulkheads and I've been cutting rod stock down to 3" to make some 38mm Av-bay harness attachment upgrade kits:
The kit includes what's in the photo above, 4 3" threaded rods, 2 bulkheads and 10 additional nuts.
Shown above is what I like to do for threading the harness through the bulkheads and attaching a BP charge holder. The charge holder is an Al threaded spacer with one end drilled out to the ID of the red ematch plastic covers. A counter-sunk 8-32 flat head screw fits nicely underneath the bulkhead to attach the charge holder and seal the bottom end of it.
I decided to make this solution more accessible for everyone, so I did a production run of harness attachment bulkheads and I've been cutting rod stock down to 3" to make some 38mm Av-bay harness attachment upgrade kits:
The kit includes what's in the photo above, 4 3" threaded rods, 2 bulkheads and 10 additional nuts.
