Gorehound:

Small pieces of fibreglas screening can be obtained usually gratis if you ask for trimmed scraps when the guy is repairing 'em. All you have to do is ask, and have other stuff you are buying in hand, and they'll usually give 'em free or bag 'em and charge ya a penny as a nominal fee.

If you use the fibreglas stuff (it takes primer & paint just fine) and can be edged with simple Micropore paper tape. Its that white or tan tape that is one of the two main ones used in hospitals (the other clear-breathable plastic stuff is horrid!) In a pinch, put the paper tape over the aluminum, then paint as usual. The tape ends up a bit fuzzy, so sand off the fuzz and repaint.

Lots of places carry both the Aber and Tamiya grill sets, you just need to look about.