I made this one a couple of years ago out of 3/4" lexan. I cut it out with a scroll saw so it was a little rough. I've had it on 3 different basses and it affected each one different.
I started with it on my King. It didn't changed the sound enough to notice a difference. There wasn't any point in having it so I took it off.
It's on my 56 kay now. It made a noticeable difference with this one. It made it a little louder but got rid of the dead Kay thump and gave it more pronounced, even tone.
I've cut a couple more out with a cnc router. They're more of a traditional shape but with 1/2" lexan.
I'm not a luthier and know very little about luthierie.(is that even a word?) I was going for the less mass approach with this one.
I want to try a few different shapes just to see if it makes a difference. Just so you orchestra don't burn up my email and Pms asking, I'm not selling them....yet.