3D print this reliable two-piece battery case. It's designed to have no overhangs and tolerances such that you can use any quality of 3D printer without adding supports. This design will support 10 amps of power output (much more than any USB port or cell phone charger, less than professional R/C car battery connectors with gold plating, etc.) The two hoops clamp onto 1/2 inch dowels with 100mm center-to-center spacing.

This battery pack was designed for the SCUTTLE robot
https://www.thingiverse.com/thing:3125573

The case integrates the following off-the-shelf components:

Power Rocker Switch ( SRB22A2FBBNN, found on Mouser)
18 AWG wires
Anderson Powerpole 30A connectors (WP-30)
3 Cell 18650 holder (BK-18650-PC6, found on Digikey)
M2.5x10mm screws, flat head (92010A020, McMaster)
M2.5 heat-set inserts (94180A321, McMaster)