After searching for a 58mm dosing cup for a while, I started seeing people using little plastic cups, such as in this video https://youtu.be/swKbVF30gN0
Turns out they're just little 75ml Decor containers designed for snacks or dips or whatever, but they just happen to really nicely fit a 58mm portafilter basket. They're heaps cheaper than metal dosing cups, there's 6 of them, and they have little lids. Store individual portions of weighed beans or grinds. How delightful.
And they're $2.50 off their 'regular' price just to sweeten the deal. Frugal as.