ok, open question then...

With costume:
What about some roman senator (start of era period) ? (some white sheet should do)
A monk robe could lead to some inquisition-scene revival/witch trial ?

Without costume... a business suit ?
You could play Bill Clinton, if you have a round room...
A security guard with a black suit and black glasses... (and some muscles ?)

Without any formal costume, it might be hard to use the costume as the base of an impersonation.
But haven't you even some 'traditional' garnement (If you ride horse, you might have some tight pants, with some boots: let's impersonate a horsebreaker/trainer).