[Help] Big Dog, small truck, help?

I just got my pup on Thursday, and I know I will need a travel crate for my Great Dane (just turned 17 wks, 41 lbs as of his vet check up on Friday). While I've been with my parents we've been fine, they have regular cars we can borrow and throw a blanket down in the back. However, I will be moving out soon into my own place (we are just in transition here) and I drive a small truck (a Mazda B2300 if it matters- small, single cab, etc).

He does great riding in the back seat of the car, but my truck has no back seat (or any space behind the driver/passenger seat for that matter) and it isn't safe for him to sit shotgun, especially as he gets bigger. Never in a million years would I put him in the bed of my truck "loose," so I know I need to get him a travel kennel, which brings me here.

Does any one have experience/suggestions on this? I haven't found much online about it (there seems to be a 50/50 split on thinking that either every truck has a back seat, or that a dog is fine to be loose in the back). I know any crate/kennel would be big for him right now, as he's growing like a weed. I know the situation isn't ideal, but it's where we are right now (in the next year or two I plan on getting a car, but that requires some saving up and isn't an immediate option). Anything on Amazon or that I would be able to pick up locally at petsmart/petco would be especially great.

Thanks!