I made my own about 10 years ago, made it just about 1/2" narrower than the distance from brakelight to brakelight.
Two inch steel stock to slide into the hitch, made a frame for the bottom out of 1x1 square tubing, around 60" x 30", I believe. Made an actual box out of 1/2" plywood to sit on top of the frame, and bolted it together with stainless steel bolts. Once I had it mounted securely, I made a lid for it that fit over the top (like a shoebox, actually). That way, if the gear I needed to haul was over the 24" depth of the box, the top could slide up (I made it 12" deep), the top still kept everything dry. Ran a chain from the back, over the top, and through an eye-bolt on the front, and locked it with a padlock.
Later, when I realized it was a ***** trying to pick 50+ lbs of luggage up over the lip, I cut the front side of the box, and put it on hinges with a couple of slide-bolts, low enough that the top still would fit all the way down when needed.
Worked great, too! Carried 7 large suitcases when we went on a cruise with my wife's family (4 more in the back of the Expedition), was able to remove it and push it inside after the seats were folded down, saved us on parking fees at the airport for a week. Carried wood in it when we went camping for 3 days at Darien Lake amusement park, carried a push lawnmower and gas in it (as opposed to towing a 5x12 trailer), and more stuff I can't remember.
Alas, had no place to put it when we last moved, wound up outside in the yard. Hadn't used it in about 3 years, gave it to a neighbor. He rebuilt the top (it warped in the rain), and is using it daily for his side job mowing lawns.
Even better - I think I spent $45 on the steel, and about $50 on two sheets of plywood. The chain and padlock I already had, bought the piano hinge and slide-bolts for $15.
