You will be better off if you can strap it down without the suspension coming into play. I experimented with several strategies before I found one that works for me. I pull from both sides of the frame on the front to a tie down point under the center of the rear of the cart. Rear straps pulling forward from the bottom u-bolt brackets under the rear axle to a center tie down in the front. The cart has never moved on me like this.
Anytime you can cross your straps it will be more secure and not subject to coming lose from movement. The proper procedure for securing mobile equipment is for the straps or chains to cross from left side of vehicle to right side of trailer and vice versa. The straps work against each other to prevent the lateral movement or bouncing that allows them to come lose.
