Some iron bars could help SAMs escape from aerian missiles. Mounted as a solid structure and whit high height could prevent missiles to explode near but at few metters away where arent so effective. Maibe a plastic cupole.
Cupole means dome if you dont know the word althought dome does not sound well to me. Ofcourse dome that protect while moving cause stationary i dont know how. A plastic dome let the radar waves to pass and in.
Gunpowder don't burn faster in closed space. Confinement in closed space increase the explosive power of gunpowder. That effect is more pronounced for slower burning gunpowder like old fashion black powder, for modern faster burning gunpowder the effect is less but still there. How fast the gunpowder burns in more related to surface area ratio. For example, more finely ground gunpowder will burn faster. Think about a piece of wood, if you chop it into smaller pieces, it will burn up faster because the surface area is bigger.
That basic rule is also true for solid fuel rocket. Surface area determines the burn rate.