I haven't ever used the RD75. So, I can't offer much except the following:
Measure the driver in open air, then measure it in your baffle and see the difference. That's going to tell you what your baffle is doing. I made the mistake at first of assuming the baffle was causing some problems when it was really the problems inherent in the driver.
I have yet to experiment with an open baffle design since I don't have a room big enough to accommodate one.
And that looks like a cool speaker that you're making. Keep me posted on your progress!