On the fly battle maps

Can you use On the fly battle maps to create cavern/dungeon walls and such to create a temple/ruins. It is good for setting up a background of that but can you easily add in walls and such to create map over the top of the background drop?

No, all OtfBM really does is add an overlay—entirely based on a grid—to a background image. It doesn't create walls, but can use fog of war to fill or reveal entire grid squares.

Instructions for OtfBM

I've never done it, but I think it can be done, although I'm not sure it's "easy" given the format. Check out the Objects and Structures sections of the instructions for details.

Example:

You can create walls, but they are all straight lines. If you want a cave wall, I recommend using something dungeon scrawl or an image you like as the background hosted on a site otfbm says work well and rely on that

Searched and located appropriate backgrounds. Leave walls to another day.
Did learn how to do War of Fog area's.