Parametric battery holder with a mounting option of built-in pegs (to penetrate and glue to a soft backing like foam) or holes can be toggled to enable mounting with a screw on a hard backing. STL files provided are sized to fit Eflite LiPo 1S 3.7V 150mAh battery.

Use the STL files as-is if they work for you, or edit the OpenSCAD file to change the parameters. You can choose a peg mount (for soft backing with glue) or holes (for hard backing with screws).

Cut a rubber band and loop through the rubber band ring. Tie rubber band at a length where the battery will fit snugly in place.