Our previous Golden had such bad hips and hind legs even a box like that would not have worked. I bought a
Petstep ramp to use to for her to get in/out of the back seat of my truck and in/out of our 5er. Very sturdy and stable, with good non-slip surface. Worked great. Now that she is gone, we use it for our lastest rescue, another Golden. Also use it to get in/out of our Expedition.
I did begin with a telescoping ramp. But the surface was a slick, thin, carpet like material and she did not like it because her feet could slip a little if the ramp was at a steep angle. But she had no problem with the Petstep.