Custom Cowl Hood Flapper Valve

[Clsccmro]

I've seen some of the cars over at lateral-g and at shows with taller than a 2.5" cowl hood with flapper doors or some kind of insert for the cowl. I made a steel cage hood with a fiberglass cowl, but now I want to put some kind of insert in it so dirt and water can't get in. I can't find any flapper valves big enough, I have a 4" cowl and everything they sell is for a 2.5" cowl. I was thinking of making something, but would like to find out if they make something already for this. It doesn't have to be a flapper valve, but it needs to keep stuff out. If you have any thoughts or ideas it would be greatly appreciated.

Thanks ahead of time.