There are lots of options depending on your whims and the number of times you expect to use your box. Many who do not plan on breeding often use the plastic toddler swimming pools that come in various sizes. I use a wooden box. Cost to build is about $50 if you have the tools; drill and jig saw. Get a sheet of 1/2" sanded plywood and cut 4 ea 2 x 4 pieces (many stores will cut it for you if you ask). Also get a pair of hinges and a couple barrel latches and a 10' length of 2x2. If you want to add pig rails, get a couple sets of closet pole brackets and a length of closet pole. On one piece of the plywood carefully cut an 18" wide by 12" deep notch in the center of the boards length. Sand all edges of the cut plywood and the cut out piece. Re-attach the cutout using the hinges and barrel latches. Cut the 2x2 into 2 ea at 24" and 2 ea at 30". Screw the plywood to the 2x2s to form your box. The higher 2x2s give a place to attach a heat lamp if needed. If you want to add the pig rails (I had them when I first started but never felt they were that useful but more in the way) attach your brackets to the front and back panels about 8" up and out from the corners. Cut the pole to length and then slide pole into the brackets. The cutout door can be left down until after weaning and then raised to contain the pups in the box. Depending on the size of your room and the size of your litter, you may want to extend the sides of the box to 6-8' after weaning as the pups become much more energetic. Or you may opt to just substitute an ex-pen at that time. For the floor cover I use EZwhelp pads
https://www.amazon.com/s?k=ezwhelp+washable+whelping+&+puppy+pad&ref=nb_sb_noss. These will just fit under the 4' x 4' box. You will need to double them up for a larger box.
Or there are many commercially available options that can be found using a search of the net. Just remember that the pups teeth begin coming in at about 4 weeks and from then on they have a tendency to chew stuff up. On my wooden box, I occasionally need to change the 2x2 posts, but the front panel is the same one I started with in 1995. I have left the wood on my box raw (untreated), but I know of some who seal the wood with a vegetable oil coating to make for easier cleaning.
Picture of my box in use