Most clubs allow for hatches, etc which are existing equipment.

Though the Tamiya apple sits high enough that it's not blocked by most, unless you using it on an early T-34.

That's the downside to metal, plastic can be easily altered so the hatch is removable if desired.

In terms of the emitter, It needs to be placed in a tube of the width of the bulb and a depth of 8mm I believe, hopefully someone will correct or confirm that.

I generally mount them in a tube in the gun sight or coax MG so it moves with the barrel. If you do that, just be careful that you're not blocking the signal with the barrel. I had one mounted in the Pz IV coax, but with the jut out in the mantlet, it would limit the signal on the left.

You could also put your emitter in the barrel, but with anything IR, it depends on where your fighting. Clubs frown on barrel emitters because you could then poke your barrel though an obstruction and fire without being hit.

If it's just you and friends, do what you want, I'd say leave the cupola as is. Turning a turret 45 degrees is not realistic as a defense, so why would a hatch obstruction be?

Our backyard setup is much stricter than club rules. I set the IR beam spread as a tank width at 30 feet. It's a great disadvantage at a meet, but much more tense IMO.