I plan to make hold hold-downs out of a pair of ½” diameter threaded rods and an angle bracket. The angle bracket will have slots to engage the collar at the top of the tank. I’ll use a pair of wing nuts on the top of the brackets along with a pair of retaining pins to keep the wing nuts from vibrating loose. The bottom of the threaded rods will be attached to the C-channel trailer frame with 4 Nylox nuts for redundancy.
Here are some drawings to help you visualize the assembly. Any comments or suggestions would be most welcome

