Here is the sliding part commonly used for the chest straps on hiking rucksacks. I lost one as the slider rail wasn't properly terminated on my rucksack. The internal diameter is 3.7mm, the same as the slider on the opposite strap. This corresponds to a rail of about 3.2mm diameter (as measured from my rucksack.
I printed this part successfully on a RRP Ormerod 3D printer, with the cylinder pointing upright and with an 8mm brim holding it secure.