I'm about to start a project similar, but a little more intricate. I'm replacing the bottom of the cupholders with plexiglass and adding a little plexiglass ring on top of that so it's more side-lit than botton-lit. Here's a 3D render of what it should look like:
The top, bottom, and sides of the octagon are layered in foil tape in that render, represented by the thin gray-ish objects, and the little white-ish dots are LEDs. I'm also considering putting some sort of decal stencil on the bottom and making that bottom lit. Also split on using 8 or 4 LEDs per cupholder, but since I'm getting 200 of them I figured I might as well just throw a bunch in there.
