Make a good square cut, right at the top of the transition (just below the rounded corners) to take the top surface off. You probably want to print the Eggfinder mount before you commit in this way.. It's probably helpful to remove the built-in shock-cord leash hasp from the 2-inch side. Carefully mark and drill the 5 holes for mounting screws. You can do a basic "countersink" with a large-ish drill bit. or with an actual countersink tool. The transition plastic isn't deep enough to do a full countersink, but do what you can. I use tiny flathead philips wood screws to attach. If you have a compatible adhesive, you can try that instead. Make sure your wires are pulled through and work before you adhere it. You will likely want spacers between the board and the mount. Try my "Spacers on a Sprue" located on YouMagine. I'd make them maybe 30% thicker for this purpose. You may need to drill out behind the board for the screw switch. I did. Do so carefully to provide clearance for the screw switch to operate. I printed the Eggfinder mount at .2 layer height, 20% infill. The screw holes are ~3mm and are non-threaded - I cut threads with the placed M3 screws. The holes could be a teense tighter - YYMV. On the 2.5-inch side of the transition, I mounted a terminal connector for the charge. I attached it with plastic push-rivets (4mmx6mm). I wired the terminals to the wires from the Apogee on the other side of the transition before I mounted the terminal to the transition. I used screw terminals :"KeeYees 5mm pitch 2-pin PCB mount screw terminal block for Arduino" - ASIN B07H5G7GC6, bought from Amazon. This mount was designed specifically for this terminal. I'm not certain others will fit the mount. It's a tight force fit. Ensure the rib on the connector fits in the slot in the mount body.