Let me emphasize Kev's suggestion about "packing the sand tight". Pour some sand in and using a tool of the right diameter, tamp it down then add more sand, tamp etc, etc until the pipe is full and "packed solid'. The more compact the sand the less chance you'll have of kinking the pipe/tube. If you've ever seen the old movie, 'Flight of the Phoenix', the original version, they show this method during the building of the airplane.
There used to be a product called Cerro Bend. It was a lead allow that melted at about 190 degrees. You could melt it in boiling water then pour it into whatever tubing you were bending then cool it with cold water. It was like bending a solid rod. When finished you only had to submerge the tube in hot water and the Cerro Bend would melt and pour out leaving no residue. It was probably a victim of an EPA ban on lead so it might not be available anymore. I haven't seen or used it for many years.
Tom