A rear-rack trunk bag can actually work really well if you control vibration, and the DIY route you’re considering is a perfectly legit solution.
For what it’s worth, I ended up doing something similar: I lined a trunk bag using some
tool box foam inserts I had left over from another project. It’s easy to cut, holds its shape, and you can carve perfect pockets for your body and lenses. The EVA-style damping is surprisingly effective road buzz and small bumps get soaked up nicely as long as your rack and bag are tight and nothing can bounce vertically.
If your trunk bag is reasonably stiff and you float your gear inside a carved foam block, it’s honestly no worse than riding with a camera in a backpack. Bigger hits are still a concern, but normal city riding, curb drops, and mild off-road haven’t caused me any issues. So yes totally doable, and prototyping with foam is fast. If your build works, Lowepro would probably love to see the idea.