This battery tray for 2x8 AAA and 6x6 AA cells is designed for infill-less 3d print, possibly even vase mode – its contour above the bottom layers can be printed as just one continues line. You may want to set the thickness of that one line appropriately; I've printed the example with two outer shells of 0.6mm each, 7 layers bottom, 0 layers top, 0% infill.

The geometry is symmetric, you can use two of them as clamshell with a rubber or velcro around it.