Here are a cople views of the alt bay. First, and outside view: (Please note, the black twistie in the picture is only in the pics so nothing rolls off the desk!)
It's a 4" coupler, with #8 all thread to hold the sled. The screw in the picture is to attach the altbay to the payload section (I just put in the screw so I could ID the hole - the screw will actually be thru the payload section BT and into the coupler) (there is another 180' away), and the other hole is for access to the switch and to vent the alt bay. On each end, there is a small hole to pass the ematch directly to the altimeter in the bulkhead. The nuts/washers on the aft end are epoxied in place. Access to the altbay will be from the nuts on the fore end.
Now here are the guts:
I'm using a Missileworks RRC2+ altimiter, with MissileWorks 38mm Li-Ion sled (and switch!). You can also see why it uses bulkhears that are the diameter of the outside of the coupler. Using a dremel in a dremel router/shaper table, on 1/2 of each bulkhead,I created a lip so that it acted as a cap and would center the bulkheads in the coupler. To do this, only trim a bit at a time, until it fits, and then cut the second with the same setup. Drill the holes in the bulkheads for the allthread (1" apart). Like I said, I epoxied the nuts/washers/locknuts on the aft end, but the fore end is secured with a nut/washer/locknut, and when cutting the all thread, make it long enough!
You'll notice the altimeter is pushed towards the aft end, and secured with a nut on the all thread. This does two things - it insures it's aligned with the hole for the switch, and it also provives room in the front for the t-nuts used to hold the altbay to the payload section.
Get the bay built, but do not drill holes for the switch and t-nuts yet When attaching the eye bolts (I used machine thread and not wood screw eye bolts. with a flatnut to hold it).
Size up things and mark on the coupler where the switch hole needs to be, and the space where the t-nuts/screws can be(make sure that the hole for the tnut isn't too hight, as you need it to fit (maybe trimming the flange) so that the fore bulkhead will still fit. Mark the center of the coupler, and then use that as a reference make to mark the BT of the payload section for these localtions (the coupler/alt bay, will be 1/2 in the payload bay, and 1/2 in the main BT). Extend the marks for the screws and switch abound the BT, and draw lines at 0' and 180' along the entire length of the payload BT (you want these marks at the nose cone - more to come later!)
Without the sled and the electronics, put together the alt bay, and slide it to the 1/2 way mark. Tape it, and the drill holes at the location for the tnuts (#6 screws) That way, the payload BT and coupler are alaight. Make one side of the BT and coupler so that you can align it later in the same way.
Now, #6 tnuts will be epoxied inside the coupler. Take the alt bay apart and clean the 2 holes. Then using the tnut figure out of part of the flange needs to be trimmed to allow the bulkhead to fit, as well as what space might be needed to allow enough thread in the t-nut (I used a small washer for this), and epoxy the tnuts on the inside of the coupler, using the Vaseline/screw method described above, When all is setup, grind/cut the column on the t-nuts until it can fit inside the payload BT, and using the mark make earlier, align the tnuts to their proper hole,a and use two #6 screws to make sure everything aligns...
Now, drill the hole for the switch/vent at the location marked before, through the BT and coupler.