ash pan or tray mod?

I wouldn't worry about it too much unless you're trying to smoke in windy conditions.  Test it and see how stable the temps are.  you can always put oven rope around the edges where the tray slides in for a better seal.  you can also use high temp silicone to make a custom seal (search for that here on SMF -- you'll see how it's done).  I doubt this will be much of a problem for you, though.

Happy smoking, and don't forget the Q-view!