Since the indestructible glass glitch doesn't work anymore, there's no way of protecting your vehicle or plane from enemies.
Of course, it's not going to be indestructible, here's my idea:

The Armour piece is modifiable just like the fuselage piece. It should have a "Toughness" slider, where you can choose how many rounds it can withstand. But we have both cannons and machine guns, so I think that if you choose the armour to withstand 1 cannon round, we should multiply that by 1000,having it withstand 1 cannon round but 1000 machine gun rounds. If you chose it to withstand 2 cannon rounds, 2 cannon roundsx1000=2000 machine gun rounds. when the armour block is hit by, for example 1 round when it's set to withstand one, it would be better to have it absorb the first but get penetrated by the second one,so on, like:

Set strength :3 round/s (can actually withstand 4 rounds, because 3 destroy the armour but don't penetrate but the 4th, not having anything in the way, penetrates)
It would also be nice too have a "Armour broken" text appear when the armour is, in fact, broken.
I might be asking too much so decide if you want to add this part if you will ever add the armour.
The armour could ricochet bullets if they're fired at a certain angle or the armour is steep,and a text saying "Ricochet" for the attacker and "Bullet Ricocheted" for the attacked