MXB2/3 uses some nonstandard low profile caps, with weird off-center stemmed Caps lock key.
You have to use an extra 1.5U cap on it (Tab or winkeyless Ctrl/Alt), as it is NOT compatible with either the standard center stemmed Caps, or the Cherry styled stepped Caps.
My MXB2 with a spare winkeyless Ctrl cap:

(white Gateron PBT set from Shadovved, Cherry profile, NO ISO support!)
MXB6 uses medium height Cherry profiled keys (just like G80-3000, G80-1800, etc), but it uses center stemmed Caps, so you can put virtually any keycap set you get on it.
Sometimes you can find blank PBT sets on Massdrop with ISO support around ~40$.
Like this one:

(Unknown manufacturer, Cherry profile, PBT.
Quality seems to be better than my previous Gateron set, but they messed up the color of the extra 1U keys needed for ISO.

)