
I have toy breeds, and use 42" puppy crates, but I've never had a bitch that wanted to whelp in one! Once pups are born, I put the bitch and pups where I want them, and have never had a problem.
I whelp my bitches on bath towels with as much room as I need to assist if needed. My crates do have two doors, but I buy the 10 packs of cardboard boxes straight from the factory (don't fancy supermarket boxes, and not knowing what they could be carrying.)
I get the 24" x 24" x 18" which means that I can use a heat pad for half of the box, under the vetbed. I do get the double thickness cardboard, so it is fairly sturdy, and I've never gone through a full pack for my bitches. I burn them when I need to replace. I can make the box fairly closed, with just enough room for the bitch to come out, and the crate is left open for her. I do cover the crate back and sides, as I find the bitches are calmer if they feel "safe."