FreeCAD (free cost, not cloud, open source) - powerful but lacks UI polish of many paid-for/closed CAD packages. Very much worth persevering with IMHO. Includes tools for CAM and CFD, with python scripting interface for customisation if you want.
OpenSCAD (free cost, not cloud, open source) - sometimes preferred for parametrics and those more comfortable with programming than CAD.
Alibre CAD (paid-for, not cloud, closed source). One of the more popular options when Fusion 360 and Onshape’s licence tweaks get too much to bear for people.
Fusion 360 (free for many, cloud-based, closed source)
Onshape (free for many, cloud-based, closed source)
Other useful links:
To start with, I’ll just leave this link to the enclosure tag on Hackaday.com where there is a wide range of examples featured.
Here’s a NASA resource on electrical isolation and chassis grounding.
Computer Aided Machining (CAM) Tools
As mentioned, FreeCAD has the Path workbench for CAM.
laser.cut is a cool looking openSCAD plugin that can handle other subtractive processes such as CNC routing.
I don’t plan to list every slicer out there for 3D printers but kiri:moto stands out for running solely in your browser (but on your local machine - it’s definitely not cloud) and supporting fdm, laser and cnc routers.
Design Tips
For air flow, whether forced (by e.g. a fan) or natural ventilation is always improved by larger slots/holes. Realistically, when you don’t want screws/fingers etc getting into an enclosure you need too keep sizes down. @kvk recommends going no smaller than 1mm slot width. But go bigger if you can and you have heat to deal with!