Recently during a Ravelry discussion regarding things to make with felted sweaters I mentioned making boxes out of the really hard felted sweaters. Here's a few pix of some of the boxes I've made.
In the first picture is a 4 inch box and a 6 inch box. On the smaller one you can see that I did a little bit of embroidery (such as it is). The little box has a thin row of plastic canvas around the upper edge, to help keep the shape of the larger box I pushed a wire through the top hem. Notice the seams on both of these are on the outside. I like it this way, it give a very angular edge to the box.
The last two pix are of the the same 6 inch box pictured above just turned inside out with the seams inside. I kinda like it this way too, it lends a softer more sculptural element to the angular cube.
The only support in either of these is the plastic canvas/wire along the top edge, there is no support in the sides nor in the bottom. Obviously the factor that determines the need for additional support is the firmness of the felt itself. I prefer to use wire, I have plenty of copper wire around here, a wire coat hanger could be used just as easily. Supports could be hand sewn into the seams as they are made here, but it's not necessary. When the construction is one piece, as opposed to the 5 pieces these were made of, the faux seams at the edges actually lend a bit of support themselves, but if need be, a wire can be pushed through those edges too.