In the current version of Designer, you are not able to specify material thickness.
For U and L Shape enclosures, the default material thickness is 18 Gage (0.048"). For a rackmount, this is the same for 1U to 4U. At 5U and up, the material thickness increases to 16 Gage (0.06")
When submitting a file to production, you may include a note stating that you would like to change the material thickness.
However, when designing cutouts, the standoffs are pressed into the material, so for example, if you are using a half inch standoff, the standoff will be half an inch from the outside bottom and does not change from one material thickness to another. The inside dimension would, however.
Note on Cutout Placement and Material Thickness
Moderators: Colin, dronne, mpyne, akirk, mrodden