I dont have a Cub, but i have 2 of the size u intend to use under slung on the rear of my Jayco outback, have been there for a few years now, and no probs, in fact i sercured mine the same way as u intend to.
Just to correct myself, ive been out back and a quick look, mine are 100mm in size, but still sercured as u mention.
Whilst i havent dragged my Jayco up the OTL, i have defo had it in the dirt from time to time, and they are all still in place.
Hope this helps.
Swanny