well have been doing it this way for a while!!
and so far have not lost a quad yet!!
self do it this way, as in pic
one 4ton strap for each side of the quad
have chain links where the strap gets put through, each link is welded to the traylor for the quad, wheel size and position on the traylor!!
this way the suspension is free, also had a concern about compressing the suspension!! even though those concerns might be misplaced, opted for the way I do it, as in pic!!
only risk is if one has a tyre going flat!!
however that is why there is one strap for each side of the quad!!
and for the long trips, normally put an extra strap at the bottom for just in case, quiet loosely, just in case it all comes loose, maybe paranoia of loosing a quad on the road!!
hope this helps!!