Alternatively, a few minutes of design and a quick 3D print yields a similar functioning pyramid base for just a few cents worth of material. Not professional injection molded quality, but you never see it once pinballs are on it. This one is 2mm thick, but I could probably reduce some and still have it be strong enough to support the pyramid.
It is a nice idea you came up with, so I don't want to step on your toes. Just made a couple for myself, but did want to share the 3D file with others to make their own and maybe offer to my Getaway group for the cost of making/shipping them. If you think this will interfere with your project, just let me know and I'll not offer them to others. Also, always happy to give you credit for the idea.
I've also already designed these other pyramid arrangements in addition to your original 4x4 base (30 balls): 2x2 base (5 balls), 3x3 base (14 balls), 3 base triangle (4 balls), 6 base triangle (10 balls), 10 base triangle (20 balls).