Looking good but I would have added another member to the top of the black frame work to support the underside of the white cross members. Not too late to bolt something in and shim or epoxy any gaps.

Fixed float is fine for the screws but I'm thinking of fixed fixed on my long axis (as per Boyans machine) to reduce a slight buzz / vibration when the machine is towards one end. In theory you might worry about heat and expansion but for the amount I use my machine and the speeds etc I think it is a low risk. Not sure yet but might make it with a spring preload . . . Or keep it simple and lock it in place.