For easier software, for quick and dirty little edits, I’ve used Tinkercad.com (but did that on a separate computer browser, just in case). In it I kinda mostly use positive and negative shapes to sculpt a thing (there are only few other tools). I basically created a block (new backside) to which I used the original model to remove material (L5 side of it) and then slabbed a copy of it on the block to get the connecting side. Freecad, Blender and others are useful for more complex stuff but are too specific and too multitools for this kind of drafting.
If you work on a logo, it does have an import function to bring over vectored images (use for instance Inkscape to make one from a bitmap image), which you can then use as silhouette to create 3D shape. Text tool only has a couple of font options, so you may want to bring any logo/brand specific texts as vector images too. At one point I was tinkering with doing similarly a Tux-case but got bored but may do a case with “Librem 5” text on the back as heat went holes, or something else on the back later (maybe there should be a text explaining to anyone seeing it that it’s not iOS or Android? 
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