Zaph SR71 kit with concrete baffle

I've been working on this on and off for a couple of years.  I had to learn how to use a router better, how to cut holes with a  jig, a bit about concrete, and after a bunch of tries I've got the front just about finished.  I'm mounting the drivers inside oak rings because I didn't think it was a good idea to try to attach them directly to the concrete.  I'm gonna use some kind of adhesive to hold the rings in place, and then also glue the baffle onto the box.  The initial plan was to then put ceramic floor tiles on the rest of the box to make what will be about the most dense, unresonant box possible I think.
You might try mixing Styrofoam balls or vermiculite with your concrete.  The right blend will maintain the density quite well and the drop in weight can be dramatic. 

Very interesting. Are you going to make the drivers flush mounted? You’d want to avoid all recesses and gaps from the driver cutouts to the oak rings on the photo as it is, to avoid diffraction artifacts.
Yeah, they're flush.  The woofer is flush with the ring in the picture and I did the tweeter since then.  The baffle is ready, I just have to get back to this project.  I ordered some epoxy resin and I'm gonna try to make the other one where the drivers are mounted on cast resin rather than the oak rings.  This is very experimental.  I want to learn how to work with a bunch of things and am using this as an excuse to do it.