I was doing the frames around the doors in my workshop. I needed to do 45 degree miters and I was constantly struggling with holding the tape measure. So I made this thingy. It works great for the acute/pointed end of the miter, but you will probably still need to grow a third hand and an elastic arm for the obtuse part to work really nice.
I have seen something like this quite a long time ago, so I will not try to take credit for the invention.