My first ever scratch build was a two-stager, and I taped the motors together and then friction fitted both motors into their respective mounts with tape wraps. Now, though, I prefer gap staging, which requires no tape.
You can have an engine block and hook in the booster, including the gap between motors that this entails, if you vent the booster.
You just need two to four small holes for ventilation. You can drill them directly into the side of the booster body tube, close to the front end (near the gap, in other words), or, if you have a large enough body tube and therefore enough room on the centering rings, you can drill or punch holes in both centering rings, so that any pressure can vent out the back.
This is necessary due to the "Barber delay," a 0.001 second delay between booster burn through and upper stage ignition.
The vent holes should be evenly spaced around the tube. If drilling them directly into the tube itself, I usually line them up directly with the fin lines. You want them evenly spaced so that you don't get any accidental sideways thrust in one direction or another, knocking the rocket off to one side.
As for the coupler, and inch or inch and a half should do it. It should be just tight enough so the booster doesn't fall off while on the pad, but loose enough that it's quite easily removable. When I build them now, I coat the exposed bit of coupler - the part that will go into the upper stage - in medium CA glue. Once that's cured, you can sand it very smooth. I like to have it about as tight as a nose cone, but no tighter. In other words, I can pick up the rocket and the booster will stay put, but if I wiggle the rocket back and forth, it will slowly start to wiggle loose.
Here, you see a small scratch build with vent holes drilled in the sides (before fin attachment):
And here are some pics from my Estes Photon Probe I built as a two-stager. In this case, I vented the centering rings, so I do not need holes in the side of the rocket. Note that I didn't have to be as careful with the spacing of the vent holes, because all ventilation will be out the aft end of the rocket.
And here is my first two-stage - and scratch build. I was so thrilled when it worked!
Now I build optional boosters for most of my kits.