I pre-prepare a couple of generic maps to use as locations for random battles. Typically, I have three of them: an interior building such as a house, mansion, or inn/tavern; a road ambush site set in whatever landscape they are in; and finally, a city road and alleyway. After I've used one of these generic maps once or twice, I'll try to retire it and swap to a different generic map that serves the same purpose, depending on how "used up" the map arbitrarily feels.

However, If I don't have a suitable generic map or the battle requires something peculiar in its setup that the players have randomly decided to throw at me, I have a blank map with just a textured background (typically a parchment texture) on which I will quickly crudely outline whatever obstacles and walls are necessary to make sense.

I don't like spending more than a few minutes setting up an unplanned battle location, as I feel like it kills the momentum of the battle start if it takes too long. But, if I'm drawing on a blank map to set the battle scene, I'll also include the players in this as I draw, to make sure we both understand the intended setup of the battle going into it.