Summary

I use epoxy or super glue to mount the two pieces together. You can use M3 screws to mount or I prefer to be able to remove the filament sensor without tools so I use this thumb screw M3 x 16 mm and 2 pcs M3 x 4mm heat set inserts...

https://www.aliexpress.us/item/3256801293488521.html?spm=a2g0o.order_list.order_list_main.201.21ef1802jiC0DX&gatewayAdapt=glo2usa&_randl_shipto=US

I use these for my bottom electronic cover plate so I don't need a tool to get into my electronics...

https://www.aliexpress.us/item/3256803162254024.html?spm=a2g0o.order_list.order_list_main.216.21ef1802jiC0DX&gatewayAdapt=glo2usa&_randl_shipto=US

I changed the way my plate mounts so I can't give you a length but I would buy the m 2mm longer than you think you need. It is easier to print a small spacer than to wait 2 weeks for a longer thumb screw.