Like many modelers, I use strip styrene to fill large gaps. However, I find it awkward to test-fit various thicknesses of strip to find the right fit. I use a feeler gauge to measure the gap. A feeler gauge consists of a dozen or so metal blades of various thicknesses; my gauge has blades ranging from .006" to .025" thick. You can combine two or more blades for thicker gaps. I insert blades until I find one with a nice snug fit, and I immediately know which size of strip (or strips) I need. You may already have a feeler gauge in your automotive tool kit. If not, you can find them at an auto parts or hardware store for around $10.
Tip submitted by:
Christopher BumgarnerBeavercreek, Ohio