I'm thinking of making a sort of drawer to slide everything in from the side but I have already tunnelled out quite a bit so the structural integrity is an issue, plus I have attached an undercarriage so weakening the bottom is not a good idea.
Your idea of a drawer reminded me of the ER-2...
Some of the old Lockheed U-2 spy planes were converted into research planes by NASA and redesignated ER-2. I got a chance to look over an ER-2 at the local airbase back when everyone was studying gaps in the ozone layer. These guys were flying high-altitude trips to the North Pole taking data. We're at 45°N but that's still quite a long, and uncomfortable they told me, round trip ride. Apparently the view is spectacular at 60,000+ feet.
That plane has much the same problem as EPO models, no space and delicate structure. The pilot has barely enough room to wiggle. The pilots talk about how they "wear" the aircraft.
To mount the bulky instrument package or cameras, ER-2's (and U-2's I suppose) have a large 'drawer', that inserts up from below, behind the pilot, occupying a big chunk of the fuselage's cross-section. The ground crew has a dandy roll-around lift just to insert and remove these modules.
I can imagine a similar miniature electronics module plugging through the bottom of an AXN, maybe into a CF box that would preserve airframe strength.