I designed this thing to help convert a battery-powered device to mains power. This design allows you to connect wiring to a d-cell shaped blank. Print as many d-cells as you need to replace batteries.
The blank has small holes in the sides to accept bare wiring. It also has a hole through the long axis and countersinks top and bottom into which you can insert a screw. The screw performs two functions: It provides an electrical contact for the replacement battery and it also secures the bare wire in place. So buy yourself a mains adapter with the same voltage as your batteries, cut off the plug and strip the wire bare for an inch or so. Insert the positive wire into the top of the first blank and the negative wire into the base of the last blank. Secure the wires with screws. I used 10 gauge x 12mm long tec screws, but you can use whatever you have to hand if you drill out the holes to suit.
I printed these things using PLA.
My basic settings were:
Layer height 0.3mm
Holes are suitable for small gauge DC wiring
Screws used in the pictures are 10 gauge x 15mm