This is not final (completed) model. However, feel free to customise it!
Video tutorial: https://youtu.be/-DLkmpBvYjQ
The shape of this arm splint is based on a custom imported 3D hand scan. The hand model then was duplicated and simplified. Simplified scan was trimmed (shaped) with "cut" command. All unnecessary mesh pieces were selected and removed from the scene, Then, the mesh outlines (holes) were selected and inverted in order to avoid perforation on the edges of the splint. Filtered mesh faces were subdivided with "bevel" command and newly created faces were deleted. Finally, modified mesh was extruded (thickness - 5 mm) and smoothed.