Direct X (.x) files can be exported by versions of Blender 2.79b and earlier, which is a free and prevalent software widely used for generating 3D models, renderings and animations. You will need to activate an internal plugin to enable export of .x files.
Blender supports import of tons of different drawing file formats, so you can use pretty much any modelling software you want/have access to in order to generate the drawing if you don’t want to use Blender as the primary modelling software. Just be sure your model is properly scaled in mm.
Once generated, you can associate the .x file as a 3D model in any .rck, .ctr, .lid, or .tml labware definition. Dragging the definition on deck will allow you to view the drawing and see if it meets your desired standards. Sometimes I just copy an arbitrary labware definition and associate the .x file I am interested in and use the deck editor as a viewer. If you wanted to use an alternative method, a quick google search will likely yield tons of free online file converters.
Regarding generating drawings in modelling software, there are tons and tons of online resources (youtube) that provide exhaustive tutorials.
Note that the data within the .x file is used purely for cosmetic and rendering purposes. The data within the file has absolutely no impact on the labware definition parameters, and cannot be used as a substitute for defining the labware.