I have made two custom colon tubes using diving speargun round latex rubber bands. They come in different diameters, the two I have are 16mm and 18mm diameter respectively. They both have a core hole of about 4mm diameter, so the flow through them is slightly constricted, however this can be mitigated by hanging the bag higher or connecting a Higginson squeeze bulb at the inlet.
I 3D-printed and epoxy-coated a nozzle tip for each which is 30mm in diameter at the business end, tapering to the tube diameter over about 40mm. This gives it a sufficiently blunt and large surface area to avoid any chance of perforation damage to any part of the colon wall as it is pushed it in.
Both these colon tubes have an insertable length of 500mm (20") and have an egg-shaped retention cuff 45mm dia at its widest point, on the opposite end. I always had an issue with inserting any colon tube beyond about 250mm (10"), but I get these two go in fully, quite easily. When I hit resistance, I simply start rotating the tube while pushing in, this gets it past any obstacle. The tube diameter combined with just the right balance between bendability and stiffness, prevents the tube from doubling over. It also helps to inject a good amount of ultrasound gel as lubricant into the rectum to start with. Also, letting the water flow once I'm past the rectosigmoid, eases the insertion process further up.
I have produced a number of custom anal tools and nozzles using 3D printing combined with epoxy coating, maybe I should post some pictures in a gallery some time.