I have always used crates as whelping boxes…. mostly because my Basenjis are very used to crates and preferred them to an open box. I use general cage wire model 400 or 500 size. I take the door off and then put an ex-pen around the crate. I cover the crate. Inside I use a heated mat covered by a crate pad and I use Bully Pads. The crate I use has custom made pig rails (made by a friend) that are removable when the pups are old enough. I find this a good solution for a whelping box with the only problem being you do have to bend and/crawl to get to the pups...gggg.... And with the ex-pen around to keep the pups in, Mom can still easily jump onto the top of the crate, on to the bed (which is right next to the whelping area) to get some time away from the pups when she is ready.
Just wanted to add that the reason that I started using large crates as a whelping pen is that when Maggii had her litter in 1994 I have a wonderful whelping box that a friend gave me to use.. it was really nice and well thought out and hand made.... Maggii hated it... and after her first pup was born promptly took him and herself back to her regular crate and refused to come out....ggg
Bob built ours - and I helped some! We used 4 x 8 sheets of plywood. The sides and the bottom are 4 x 4, the top is a frame with wire fencing so you can see in but mom can't jump out unless you leave it open. We cut a 'door opening' in one corner and attach a wire crate, then use a small piece of wood to create a low barrier that mom can step over, but pups can't quite get over until they're able to climb better. This is mom's hideaway from the babies. We put a pig rail up and also set up one corner with a low L shaped barrier to keep pups in one section until their eyes are open. We put linoleum flooring in, and as they get more mobile, add artificial grass for better footing. We actually have 2 boxes, one we made with hinges on the sides so it's easier to store and move (the solid one is really heavy - maybe casters/wheels on the bottom would help? I know I'm not describing this well, but at least you can visualize it a bit. When we set them up for this year, I'll try to get a couple of pics and post them. We use a heat lamp for the first few weeks - on a tripod, so it can be adjusted easier.
Pups are well protected from drafts and curious Basenji house mates, and I can climb in with them! Aaah, puppy breath!