House rules per the Gunny

[triangular_cube]

The AFVs are affected by Xs only in direct & bombardment fire.

The open-topped ones have to take an M2, the closed-top (turreted tanks) take a regular M check.

Additionally, if there is an 2X or 3X result, the 2nd X is taken from an open-topped AFV, if present.

And if they are alone the 1st X is still hitting the open topped AFV. It seems by house ruling to make the open top more vulnerable to MX checks you are actually making them less vulnerable to DF/BF by strongly reducing the chances of causing a step loss.