It usually had 2 full suspension cross country bikes on it.
Not the best pic, my dad's kinda in the way but this is generally the bikes that were on it neither of them look nearly as light as the hardtails you've got pictured.
did you have the bottom pads on the glass or the tailgate?
I ain't that bothered cos I use it on the golf, I was just experimenting
I have found that the weight of the bikes tips it up, I have tightened the top fastening as much as possible but still it tries to tip up, if the side fastenings were higher I think it would be very secure, if I really wanted to use it I'm sure I could alter it