The gate design is dependent on the type of camera you are using. An area-scan camera would like to see the film perfectly flat as it captures the frame. A line-scan camera like the Spirit and others use only captures a single (or three) line/s of the image, so a line-scan can take advantage of the curve.
Having said that, the issue you are trying to solve, how to capture a warpy film, can be dealt with to an extent. The gates that use a pressure plate, as in a camera or projector, can give us guidelines. The Richardson movement that was mentioned in another thread is a good example. A version of that idea would help many films. If a truly terrible film needs capture, another approach would be to make a sandwich between two pieces of optical glass, like microscope slide glass, or similar. This idea needs a stop-frame capture, as you would need time to open and close the glass as the film moves.
many choices, and maybe separate gate designs for different situations. This implies a quick-change type of mounting, so you can change gates as required.