These are strain relief brackets for the RepRap MK2B bed heater PCB, variants for 12V and 24V wiring.
The channels are 3.5mm and 3.0mm respectively, and designed for 14-18AWG silicon covered wire. The channels for the thermistor wires are designed for 30AWG silicon covered wire.
I glued mine to the PCB with epoxy, spread more epoxy on the mating surface and glued the two parts together with the wiring in place. This creates a permanent strain relief that will be difficult to remove, so test your wiring before gluing!
Generic PETG or ABS
Print one pair, for either 12V or 24V wiring, with either PETG or ABS for durability. I don't know how well PLA would work, if you try, please let me know the outcome.
Print the base 'A' (coloured yellow in the CAD image) with the wiring channels on the bed, with supports.
Print the cap, 'B' (coloured blue in the cad image) with the wiring channels up, supports are not needed.
My Cura profile for these parts is included here, I used a 0.35mm nozzle and 0.3mm line width.