That brings back memories of my first two incubators that I made myself. One was made from used redwood from an old kerosene incubator I bought at an auction. I used a heating element from a clothes dryer, plastic Rainbow bread trays to place the egg trays on and an old blow dyer to circular the air as I recall.
FYI, the newer model GQF incubators have two holes that are about 3/4\" diameter. Both are in the back center with one near the top and one near the bottom. Those incubators are about 8 3/4 cubit feet, so you can figure from there for starters. The locations of the holes in your incubator will depend on your fan location and the direction of air circulation.