I use the 'slide in bay' method on HPR, there are a few issues including;
1) vents
Not a big problem, I put holes in both the body tube and the 'bay tube' line them up not a problem. I make the holes larger in the 'bay tube' to make sure it lines up.
2)Attachement
How do you keep the e-bay where you want it in the body tube? I have screws that run thru the body tube and screw into the e-bay. I use floating nutplates to make is simpler to line up. I'm building a new slide in e-bay, for this one I am using a hard stop bonded into the tube plus screws (only can slide in from one side)
3) Sealing / Blow By
This is one of the harder problems, how do you keep gas from one side of the e-bay getting to the other side (between the body tube and e-bay). I have been using a 'flange' of PVC tape, the kind you use to wrap gas pipes. It is conformal enough to fit in the corner, It works but does have a bit of 'cheesiness' SP? to it. On future ones I plan to use a neoprene gasket or even an o ring. I'm sure I am over thinking the blow by issue, I worry about two things, the 'pulse' passing by the barometer holes and causing problems, and the corrosive nature of the BP charges causing problems with the electronics.
4) Switches
I offset the e-bay tube where the switches are and have a mating hole in the body tube. Used this with slide switches and also key switches. On my new e-bay I am installing floating nut plates and forming a lexan door, cutting a square out of the body tube that will be filled in with the lexan. This way i can see the switches but have a flush surface.
If you are interested I can dig up some pictures of the last one, i should have the 'new improved e-bay done in a week or so.