Lee, the tongue box was a cheap harbor freight special
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=66244 .
I paid $80 for it and didn't need it shipped because they are local to me. I don't know the dimensions off the top of my head but you can find them on the link. It fits nicely,works nicely and is inexpensive.
I threw the trailer together in less than a week to do a camping trip.
-A quick summary since I never did a write up on it because it wasn't going to be a long term set up.
The m416 was a cheap craigslist find with straight rails, lunette,landing leg, functioning hand brake and little rust!
I wire wheeled the entire frame and tub, primed it and spraybombed it with either rustoleum or krylon "hammered black". I liked it, it's textured and hides blemishes. I rhinolined the inside of the tub. The previous owner had cut out the back of the trailer, never added a tailgate or reinforced it. It makes the tub flimsy.I cut a peice of 3/4 ply to fit, drilled a few holes added few bolts and rhino lined it, then bolted it in place of the opening and caulked it, this made it easy in the future to unbolt it and do it to the rite way.
The lid again is wood, I took some 2x4's and a few peices of ply and sandwiched them together with some gorilla glue for strength. I stained it with a cheap deck stain to keep the water out. I cut the lid to be larger than the tub, that way I could add some weather stripping and keep the weather out.
The gas struts are junkyard ingenuity,I snagged them off the rear off a jeep cherokee and doubled them up with longer ones off of a kia van.
The hinges were cheap hardware store door hinges put together with lag bolts and stainless hardware.
The rack is from a nissan quest, with aftermarket yakimah mounts and crossbars(junkyard find $20). It was easy enough to just lag it through the ply and 2x4.
There is no latch to keep it closed. I just used ratcheting straps to close it up.
I already had the tent sitting around so I bolted it all together and called it a day!
I did however spend time and money wisely by making sure the axle was in good shape. Bearings were in good working order so we just repacked them added a new pair of Napa shocks and were off to the campground.
I did remove the handbrake. I wasn't a huge fan of that idea but it was again a quick project thrown together. I suspect you can just relocate it and use a few turnbuckles. Never looked into it much. I never changed the tongue, it's original. Again it was a cheap and quick project. all together with trailer, supplies, tent and tongue box I may have a thousand into it.