The first part I worked on was the suspension mount. So here it is: an outer loop using thin pieces of PLA to hold an inner mounting shell. It's stiff, but I can get it to flex by a few mm without too much bother. 

I made up a mount plate for the pi and HDD as well. I've drilled this Pi to accept M3 bolts through it's mounting holes, and they tap straight into the plastic. It would take a significant amount of force to remove it!.
Normally 2.5" HDD's mount using 1/8" screws, but I don't have any. Turns out that M3 work well enough because the thread is only a couple mm long and the pitches appear to line up. Definitely a hack, but it works for me.

The HDD mount can accept two HDD's, so you could easily stuff 4Tb of storage into this thing - and there's still half of the internal volume left over for the power supply.

Next I need to figure out how the latch/un-latch will work.