one thing i'd thought i would address is to compile a list of compatible helmets for those who want to modify their army builders. it will help other people down the line who choose to dremel out helmets.

so far we've got

ferret: tiger claw

nullifier: arah ambush, most new sculpt figures.

night viper: new sculpt zartan, iron grenadier new sculpt

pit viper 1985: duke, gung ho, most new sculpt figures

h.e.a.t. viper: new sculpt cobra coils

lamprey: new sculpt firefly, maverick.

who is undertows head compatible with?

_________________

Undertow is the one that most people use to put the helmets on. No use in dremeling out a masked head to put on another head. Unless thats not what you mean by that.
Another easier thing would be to make a cast of the head instead of purchasing a head just to dremel it out. A casted head could be put in some Kerosene if you didn't have the enlarging stuff to make it larger to fit on most heads as well.

_________________

The only downside to the Kerosene method, is that the helmets/heads usually turn out a bit deformed.

_________________