There is no better way of feeling the worth and thrill of keeping your flag in pristine condition than by having your own flag case built by your very own hands. For a start, it would be wise to know the actual dimensions of the flag to be contained.
In this case, let us take one 5 by 9 feet flag, one that is usually used for burials, or official flag posts. By knowing the flag size, it would be easier to have it folded up to a triangle and have the dimensions measured for the flag case. Make sure that not only the shape of the triangle would be taken, but also the thickness of the folded flag itself.
Now, determine the material to be used for the frame itself. A good choice would be any hardwood such as oak. When outlining the triangular shape on the plank, you may use a speed square or any similar apparatus to ensure a nice looking triangular shape. Make sure to leave a quarter inch allowance for finishing or fixtures that would be added later on to your custom flag case.
After cutting the base of the flag case, you would have to consider the grain orientation of the wood. Therefore it would be wise to turn the plank as you make the initial triangular cut, and proceed on cutting the other side. Having the two triangular cuts done in opposite orientation means having the grain line up in a parallel fashion later on.
Add up the finishing and a few fixtures to the cut outs and you would be on your way to having your own custom built flag case.