There are well designed sheet metal chassis and there are poorly designed sheet metal chassis. My prior experience is that a poorly designed sheet metal chassis is most likely inferior to one built using the VexPro tubing and clamping bearing blocks and will cause you a lot of grief.
The CAD resources Jose linked would be good to study. When doing so, think about how each piece will be made and what equipment will be needed. Also, think about how the pieces will be assembled, including the order that the fasteners will be installed and what access will be necessary to install those fasteners.
I have had coworkers design stuff that the fabrication shops kicked back because it was impossible to build as well as some enclosures where “the last panel had to be installed from the inside” :rolleyes: