UPDATE
New model uploaded with hole for 5mm LED under the switch and two pilot holes in the back for external connections or mount points.

This is an addition to my ThinBoard conductive ink/epoxy series, it can also fit a standard sized small bread board with the side pegs filled off.
https://www.thingiverse.com/thing:3834264

The switch opening is designed for a 12mm x 5mm SPST slide switch to simply slot in place.
https://smile.amazon.com/gp/product/B01N7NCW8N/ref=ppx_yo_dt_b_asin_title_o03_s01?ie=UTF8&psc=1

The contact springs are covered in conductive ink or copper foil before being slotted behind the battery dividers.
https://smile.amazon.com/gp/product/B01MR5DSCM/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1

Use a flat square coil of bare wire taped with copper foil tape to a bit of old business card for the final positive and negative contacts. The contact wires can be tucked behind the battery separator walls to hold them in place (or super/hot glue). Then connect the positive wire to the center post of the switch and another length of wire to the right post of the switch (or left if you want "on" to be that direction of the switch.

Attach power and ground output wires to the back of the power rails on the breadboard by first removing a small section of the double stick tape over the rails, pushing the rail springs out the back a bit, then inserting the power wires into the gap, and pressing the rail springs back in place.

This is my first use of a no-heat solder "wire glue" - Make sure you have a good bare metal to bare metal connection first:
https://smile.amazon.com/gp/product/B000Z9H7ZW/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

Check out my 3D Printing & Digital Electronics playlists on youtube:
https://www.youtube.com/playlist?list=PLutzSUqCeqd0FMebcAsHZCR3Kk3X2P4_C
https://www.youtube.com/playlist?list=PLutzSUqCeqd11xOHDvpy5OEO6Pi6CFmWI

https://youtu.be/Dbbhqt9AT2I?list=PLutzSUqCeqd11xOHDvpy5OEO6Pi6CFmWI&t=250

XYZprinting

da Vinci 1.0 Pro 3in1

No

No

.02mm

15%

The battery pack can be printed without supports but the springs should have a brim.