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I do not put a spacer in the middle and I haven't heard of anyone seeing any positive results when doing so vs. not. The most important thing is having both sides of equal height with the keys. A spacer on one side to make it even may be needed - which a simple washer will do fine with.
There are many ways to engrave your moms name into the key holder. Try tinkercad.com and just upload the stl files. I do not give out original files of the design for many reasons. Fusion 360 can cut into the stl file using patch mode but I have not tried that with stl files.