This is just a simple lightweight wrench you can stuff in your toolbox or field tool bag to remove a Beta X-Trainer's front fork cap.  Beta's official X-Trainer fork tool is machined metal and also contains pegs to remove the inner spring, but it's overkill if you only need to change the fork oil.

I recommend printing in something stiff and chemical resistant like PCTG, ABS, ASA or PETG.  You may find that PLA is too brittle and creeps over time.  I printed mine with 7 perimeters, 5 top and bottom layers, and 25% gyroid infill to minimize stress concentration.  Mine printed perfectly to scale, but if your printer isn't super-calibrated for dimensional accuracy, you may need to increase or decrease the model size by 1% in your slicer.