The Suckit dust boot is a decently priced option if you don’t want to craft your own dust shoe.
Digital calipers as everyone suggested.
I’d get a roll of double sided paper tape such as
If you’re using it indoors and don’t have it, hearing protection. I like the 3m workmate, it has radio, mp3 player (off a micro SD) and bluetooth. Likewise safety glasses if you don’t have them.
I’m also now a GWizard convert, if you’re new to CNCing, buy a 1 year subscription to GWizard’s feed and speed calculator. It’ll help you get your pieces cut accurately and quickly with good surface finish. There’s a 30 day free trial.
Make sure you have a 1/8" collet if your router doesn’t include one, my dewalt did not.
Most of us recommend a sacrificial wasteboard on top of the machine’s own wasteboard. I like these inserts: https://www.amazon.com/gp/product/B002KSZ13G/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
I think I needed 2 adapters from my hardware store to be able to drive them with my drill. I cut the hole pattern on the CNC itself, installed the inserts, then flattened it. On that note a 1/2" or 3/4" surfacing bit will make that go much faster if you want. A bunch of 1/4-20 hardware in various sizes would help there also. This is just how I went, some people use T-Tracks or other holddown methods.
A level for … well leveling the machine.