The stl is used for the bed model in PrusaSlicer.and is installed int the Printer Settings tab, Bed Shape Model dialog. Load the stl .file Normally the stls are in the PrusaSlicer\resources\models.

The Overlay or what Prusa calls Texture is loaded in the same dialog. Normally the svg files are in PrusaSlicer\resources\icons\bed But both files can be kept anywhere.

The Overlay was created using OpenSCAD and exporting as an svg. PrusaSlicer sacles the svg internally to fit the size of the bed and on a Railcore, the correct bed size appears to be 315X300 with the origin at 0,0. To verify that the bed size is correct, place the 300mm X300mm stl on the bed as if it is to be printed and the outer periphery of the Overlay should exactly fit.