Hard cases for storing portable 2.5 inch hard drives / SSDs are quite good, such as these ones by Inateck:
(I'm sure very similar cases are available with different brands.)
The main downside is that the cases are about 35 mm thick, although that does mean you can fit two Voyager or DM42 sized calculators inside. (I tried a HP-42S and it didn't quite fit, but a DM42 is a snug fit.)
Discussion of hardware issues related to the DM1x/DM41. Please ensure that you specifiy whether the issue is related to a credit card size unit or a "Voyager" unit (DM1xL/DM41L).