A simple and well working solution is, to plug the open ends into a short piece of a bigger sized tubing. But it remains a gap between the tube endings. The disturbed blood flow at the gap leads to thrombogenic activation of the blood sample. And in case of coated tubes, the blood gets in contact with non biopassivated surfaces.

A polycarbonate tension band around the loop leads to perfect fit and sealing at the splice. Its length can be varied and adjusted. A regulation of the closing force at the sealing surfaces is implemented

clamp setup

variable slot width of the loop stand

cut strip according to the tube

kink the strip by hand

intensify the kink

fix both ends

tension ring and tubing

close the loop with a lap joint

press the loop inside the tension ring

tension regulation

Result