OpenXR doesn’t have proper support until 4.27. Newer versions of the engine have support in the motion controller component for getting the render model for the connected controller from the plugin, although I don’t think it’s in 4.17.
4.17 seems to only have C++ functions for fetching the model from what I can find.
https://github.com/EpicGames/UnrealEngine/blob/4.17/Engine/Source/Runtime/HeadMountedDisplay/Public/IXRDeviceAssets.h
Valve also has models for the knuckles available in the SteamVR installation under: \Steam\SteamApps\common\SteamVR\drivers\indexcontroller\resources\rendermodels\