Well, this didn't happen until they were 400g, so they weren't tiny anymore. They were fine for about 6 months and then suddenly, over less than a week, three of them scraped up their heads. The other two have relatively minor scuffs compared to the one pictured.
The dividers appealed to me as a temporary solution. I wasn't originally planning to use them as long as I did. You're right, though. This wouldn't have happened if I had bought a second set of tubs and attached the dividers to them permanently.
Hopefully others can learn from my mistakes!