I'm going to build an enclosure for my CNC mill to keep coolant and chips from going all over the floor. It took about ten minutes to model the mill table in FreeCAD but modeling all the aluminum extrusions was looking like a lot of work. Fortunately it isn't necessary
More and more vendors are providing CAD data in their online catalog. As long as they provide data in one of the non-proprietary formats, it should import very nicely into FreeCAD.
McMaster-Carr's catalog is legendary and many of their products have CAD data associated. You can download in a bunch of different formats that are compatible with FreeCAD, HeeksCAD, or probably any other CAD system on the planet.
Just click on the part number . In the item detail pop-up there's a CAD link that will take you to a page where you can get 2D and 3D CAD models and also see some dimension data online.
Misumi's site is maybe even better. Misumi will custom cut aluminum extrusions without a setup fee. The nice thing is they also provide custom CAD data for the parts you specify. For example, if you want a 330 mm extrusion, they'll cut it, but you can also download a 330mm model to use in your design.
In the design at the top of the page, I used three different length pieces and played with them in FreeCAD to verify that things would line up on the table the way I wanted.
I wish all vendors provided this, along with detailed specs, schematics, and illustrated parts lists. If you know other noteworthy vendors, drop a comment.