Old school alignment guys; gauge mount idea. What am I missing?

1MeanA said
Not overly cheap…I found one of these used for $100. They are $175 new on Amazon….cheaper in the US I’m sure

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I saw similar tools when I searched, not excited to add another $200 in tools to do my own alignment. At least not yet. I was thinking I could do a simple tool for much less, both ideas ended up being more than I expected.