I am thinking of building a mid-sized rocket for H-I motors with single (electronic) deployment and I am curious about where one would place the ebay and the possible different configurations that can be used.
You could build it just like it was going to be dual deploy, but just have the electronics deploy the main.
The question I have is what your flight profile looks like. Are you going to use motor eject at apogee and deploy just the shock cord (letting the electronics blow the laundry close to the ground)? Or are you going to deploy the main electronically at apogee? If you're doing this on an H or I, unless it's a very heavy rocket, wear comfortable shoes because you're gonna walk a ways...
I am thinking of main at apogee, and yes for larger rockets. For example, I launch my stock-built Tethys on H and Is with motor ejection and I am wondering how this could be configured for electronic apogee ejection instead.
Let's assume you would build the ebay into a coupler with forward and aft bulkheads. You'd probably want to cut the rocket at a location corresponding to the forward end of the largest motor you plan to fly plus half the coupler length and insert the ebay there. I'd install it into the aft part of the rocket with screws or those plastic rivits PML sells. Also install the forward airframe to the ebay the same way. The only benefit you pick up with this deployment configuration is that you know the parachute is going to deploy at apogee and you don't have to worry about the length of the engine delay. This will require a plugged forward closure for your engine.
If you are using single-deployment, connect the charge to the apogee or drogue channel of the altimeter, not the main.
With very little additional effort, you could go dual deployment. You just need to leave enough room under the ebay to have 10-20 feet of shock cord joining the engine can to the ebay. Use the "drogue" channel to blow off the engine can at apogee. You don't really need a drogue chute as the shock cord itself has a modest amount of drag and the rocket is going to fall on its side once separated. The "main" channel blows the main chute at some predetermined altitude and you don't have to walk far. If you do this, secure the nose cone with shear pins so the shock of the separation at apogee cannot throw out the main. 2-56 nylon screws are commonly used as shear pins and take around 25 pounds force per screw to shear. There are calculators that tell you how much BP to use to achieve a given pressure and pressure times the cross sectional area gives total force. Shoot for at least twice the force need to shear the pins (2 x 25 = 50 pounds if using two shear pins). For smaller rockets, you can use the square styrene plastic extrusions which take less force to shear.
Another alternative is, build the fin can with a zipperless design. That leaves a BT between the fin can and electronics bay, and another between the electronics bay and the nose cone. You can make these two different sizes and make them both so they can attach to either end of the electronics bay. Then you can use the longer tube between the ebay and fin can with the main in the lower part for apogee deploy of the main, or put the longer tube on top and use true DD when you try a J-K motor and fly a little higher.