I put electronics bays in all my rockets if I can get them in there. I fly a lot of short fat rockets so that bay often goes in the nose cone. I have done it several ways. I don't trust epoxy alone to hold in the rings. I have had the ballasts of two nose cones break loose. One during flight and one while I was moving it around in the house. You don't want your to break loose and bang around in the nose cone. Will it? Maybe, maybe not. Some nose cones are slippery and wont even hold paint very well. There are several ways I have done it.
1:
A centering ring above the shoulder in the nose. In your case it would probably be a CR for a 38mm. A little sanding will get a Madcow ring to fit well. Scuff up the area really well and epoxy in the ring. Use a 38mm MMT or body tube for the inner tube and slide it all the way in until it stops. Cut it just to the edge of the shoulder, and drill holes in the inner tube for sampling ports and switch access. These holes can be larger than the sampling ports in your nose cone. Epoxy the inner tube to the ring and on the end that goes into the nose cone. This will definitely keep the ring in place. I also notch the ring and attach a piece of kevlar or nylon the the inner tube just like I would to a MMT, to allow shock chord attachment.
2:
Uses a ring or plate above the shoulder as above. But uses threaded rod in the ring that extends out of the bottom of the nose cone. These rods hold a stepped plate or cover that seals the nose cone and effectively holds the upper ring or plate from moving. You can put the sampling ports in the shoulder. Like Nytrunner said though, make sure your body tube doesn't spin and cover the sampling ports. I've done this on three rockets, two 4" and a 3", and have never had that happen though. All these parts can be sourced from your local hardware store and wont require anything to be ordered. #6, #8 or #10 rod will be fine. You can put an eye bolt or u-bolt or whatever to attach your harness to, or a piece of kevlar or nylon to the permanent ring.
3:
This uses a ring with a 38mm hole in it attached IN the shoulder. When I do this I epoxy the ring and then countersink #4 screws through the shoulder into the ring to keep the thing from moving. Go to Lowes and get you a 1.5" plastic slip joint extension tube. It is the piece under your sink that connects the P-trap to the drain. The shoulder fits perfectly in a 38mm centering ring; cut the pipe just above the shoulder, maybe 1/4" or so. I think there is a standard hole saw (probably 1.5") that cuts the hole perfectly, I would have to double check. Scuff it up and epoxy it to the ring. Being in a 2.6" nose cone you may have to cut the ears off, and possibly the knurls, of the cap. It gives you a threaded cap as well. They make a smaller size, 1.25", that you may need for the 2.6" nose. If you are cutting your own rings it won't be an issue, just cut a smaller hole, and of course less room for electronics. I have used it in a 4" and 3" nose cone. The 3" hasn't flown yet but will use the same sled as the 4" and it has flown many times. I attach the harness to the ring and don't rely on the PVC pipe to hold any recovery force, it may hold or may not. I really don't know.
Don't know if you are a gun owner but for charge wells there are many options. 444 Marlin works well if you have any of those. Any of the '06 family works with the neck cut off (25-06, 30-06, 270). 308 works too but doesn't have as much capacity. 357 mag will hold about 2 grams of BP and a ematch but no room for wadding. 44 mag or 45 colt (Long Colt not ACP) will probably work. 410 hulls as well.
I have pictures of all of my nose cone bays somewhere. Ask away if you have any questions. A PM will probably get a faster response.
