Ok, quick test I did was add the DeepFreezer module to a part without internal model from VENS Revamp (copy of InflatableHAB). CrewCapacity and FreezerSize were set to 4.

Ingame I get the following message when clicking the UI freeze button "Cannot freeze kerbal. Freezer is full".
The UI Menu lists the freezer space being 0.

Log also spams the following error:
(Filename: Line: -1)

DF.DeepFreezer[FFF76E3E][136.62]: Part InflatableHAB2(4086808552) is loaded and internalModel has disappeared, so re-instansiate it

(Filename: C:/buildslave/unity/build/artifacts/StandalonePlayerGenerated/UnityEngineDebug.cpp Line: 56)

NullReferenceException: Object reference not set to an instance of an object
at DF.DeepFreezer.resetFrozenKerbals () [0x00000] in :0
at DF.DeepFreezer.OnUpdate () [0x00000] in :0
at Part.ModulesOnUpdate () [0x00000] in :0
at Part.Update () [0x00000] in :0
Hope this is of some use.

V0.17.0.0 supported user built parts with DeepFreezer partmodule attached as long as there was NO internal model defined for said part.
V0.18.0.0 does not support this. Request to re-instate this support.

Fixed.
0695cea