I need a really basic 3d modelling tool to essentially model a flat rectangle with: a length-long raised section on one side, with a hole throughout that section, and the whole thing split in half with some kind of joint in the middle (like snap/overlaps or simple tongue+groove joins)

This sounds like it should be "easy" but also all of the cad software are just.. either far too powerful and complex (I'm not feeling "figure out software" today) or cost money..
For anyone curious here is my terrible diagram thrown together in inkscape to show the kind of shape I mean (This is also serving as a reminder for me to actually do this)