I built a crate under a kombi bed & it was fine in the first van, as it had an open area in front of it. The dogs didn't like it when we swapped to a kombi as it was more enclosed & whippets are weird creatures.
But it's the best solution, other than the fantastic Lintran Box, but that was too high to fit under the bed.
I'm tying them up now but I worry about it. We're swapping back to a camper to have the open space back & I'm going to secure them on the floor, rather than up on the seats.