I've made a couple of tin box guitars, one a 3 string and the other a 6 string. Both of them have a through neck, and that is enough to keep the rigidity of the box. I cut the hole in the tin, fold the tabs into the box and use them to screw the tin to the box. I don't what other folks will say, but for me, no other bracing is required.
I've attached a photo of my 3 stringer. You can't really see how it's been attached, but I thought it might give an idea of what I mean. I should also point out that I made this 4 years ago. While I might do a few things to make the fitting a little neater, I can vouch that it is still structurally sound.
The only other tip I can offer is about the sound. Tins have a tendency to vibrate and make a nasty sound if you strum too ferociously. This can be totally stopped by shredding a plastic bag and putting it into the tin when you seal it up. It doesn't affect the sound at all, apart from getting rid of the vibration.
I have been playing diddley bows for around 6 months. And I love it. Its like a second wind. Any way, yesterday I picked up a 6 string acoustic for the first time since I been playing the d bow's, and played it with a slide (without raising the nut thingy yet-the frets were clashing a little). To my amazement I realised that the experience with the diddley bows had TOTALLY revolutionised how I looked at the instrument. It was no longer a complex intimidating chord shaping thing, it had become.....a sonic rhythm box instead LOL
I have made a couple of one stringed cookie tin guitars. Both I made with the neck going through the inside of the body, and then screwed at the base of the tin to reinforce it all. I didn't reinforce them at all with any additional pcs. of wood.
I used a drill bit to !carefully! bore a hole for the round neck to go into, and had to do some additional bending inward to make the hole bigger. Once I had the metal hole shaped and big enough, I had to cut a small notch in the lid so it could close properly. I attached it at the base and got a little creative with the string holster.