As I think this through Dave, you could possibly get what you need with some 1x4 and a hole saw.

Clear cedar would be a good choice because it's light.
The flat 1x's would go in first and then you would form a tee shape with the "holed" pieces on top if there was clearance.
If not they would go on the bottom.

Basically a hole-sawn piece of 1x would replace the store-bought, white molded product.