When my old thermopen broke I bought a Thermapen One but my old case didn't fit.  I printed it using Polymaker TPU90. 

To get a successful print I positioned it upright so the long edge was vertical. I used organic supports and made the interface layer PLA for easier removal.

Notes:

• There's about 2mm too much length
• This must be printed with flexible filament because the thermometer will not fit in otherwise
• It's very very hard to remove the thermometer from this case