Inspired by Masukomi's Simple Valet Tray (which I loved the idea of but is just a bit too big for my small printer, and printed small enough to fit is still not exactly what I want), I made a little compact tray that just frames the hexagon silicone ice molds I use. It gives enough rigidity that stacking up to three of them while filling and transporting to the freezer is less of a mess.

I printed ours in PETG and with no top/bottom layers thus showing off the infill, and allowing for drainage if necessary. That's just fine for my purpose (extra support while filling/transporting), but if you want them watertight, you can just print as-is. 

Threw this together in Tinkercad and cleaned up the model using the tools in Prusaslicer.