This is a tool to enable removal of radiators by capping off both ends to prevent spillage of the nasty black coloured water / sludge onto your carpets.
It is based on a commercially available product.
Threads have been modelled using BS Pipe Thread spec sheets - care to be taken to ensure 3D printer is properly calibrated for the specific material for accuracy.
Suggest printing with either PETG or ABS with dense infill for strength and longevity.