I highly recommend making a tracing. The simplest way to do this is to make your butt-print on a piece of paper to begin with (you will still have to outline the indentations, since the indentations will fade with time), but you can trace the indentations on whatever else you use, also. With a tracing, you have a permanent, portable record of the size and shape of your sitbones that you can lay over any saddle.