Making a box for a pattern bar design that involves frit or lots of small pieces is necessary and simple.
Let's assume you want to make a pattern bar that's 25mm by 25mm by 200mm long. Start by cutting three strips of glass, each 25mm wide and 200mm long. Also cut two 25mm squares of glass. You can use any colour, but remember that the colour you choose will make up the outside of your pattern bar.
Assemble the three strips and two squares to make a small box, with one piece on the bottom and the others attached to form sides and ends. To do the attaching, use a hot melt glue gun.
The advantage of using hot melt glue to make a pattern bar box is that after the box is assembled, it can be filled with frit and other scrap outside the kiln, then easily carried into the kiln and dammed as usual. The glue will burn off during the firing. You can finish the box with a final strip of glass laid on top, but this isn't essential.
You then dam the box as for any other pattern bar.