if you have a compound Miter saw on a slide this becomes a lot easier first finger joint two piceses on one side to make a 90 miter the other edges of the two pices that join to make the 90 at 45 miter the last pices at 45 on both sides
then make a jig out of two pices of sheet material(MBF, Plywood, etc) with a rectangle hole in each with a side parallel to the side oppisete of the 90 the mount the two sides together at a conveniant with apperts MAKE SURE THERE TRUE TO ONE ANOTHER! then to cut your 45 finders place the pice 45 degree down and clamp it to the jig.
h> My FIL passed away a couple weeks ago and he has a military funeral.