Thanks Bob.

I have fins in my design as well as two IR pass filters so sunlight isn't an issue. The second filter is an 1/8 inch thick, completely opaque. Once assembled, the sensor will never see the light of day, only IR.

To emulate the Tamiya, the fins vs opening has to a certain length, which is fine, I have the dimensions worked out. But if I could shorten them just a bit, there's a different design that would be really cool. Problem is, it opens up the angle a bit wider.