You could glue some change in there, it'd probably be heavier than sand.
This is going to be so sweet. I've been looking at drivers since your OP to try this in my car to see what this type of system is like.
That's a really good idea! I was looking for weights etc... sand as well. But yeah. It'll do great laying face down. I'll consider it.
I did some speaker pointing today and mounted the ring. It's amazing to hear the speakers outside of the car, and inside. Inside the car... well you get massive reflections so the speaker, or any speaker, sounds very different. I wanted to minimize reflections, but ultimately point the speakers directly to my ears. The dash/window and side glass offer a bit of reflection. I'll have to add a carpeted dash mat maybe.
I'm a believer that you never want to cross fire the speakers in the a-pillar. I never understood why that was recommended. Off axis, you lose a lot of signal... mostly the high frequency. I assume some do it to tame the tweeter, and maybe they don't have an EQ, but you get all the "good stuff" if the speaker is pointed at you. Anyway.
These are pointed to my left ear and a tad upwards to clear the dash as much as possible. Next is finding an unlucky t-shirt and begin building the pod walls.