Bike rack recommendation for front of tow vehicle

We use Saris Cycle On (old style for a 2 inch receiver) where the bike tires ride in troughs; on the front of our Suburban. With our last vehicle we used a 4 inch drop between the receiver and the rack to get the bikes lower.
The roads are so bad we have to tie down the wheels in the troughs. I wonder how you would keep the tires on the platform from slipping around.