You did a good job on the miter gauge. It is larger than mine, I need to make another closer to the size of yours.
They are a must for picture frames of any size to get a good 45 degree corner but, if you are making box corners more than 2" tall, I find it much easier to tilt the blade to a 45 degree angle and cut the pieces with a standard 90 degree miter gauge.
Give my method a try to see if it works for you. I like it because I can pretty much make the grain match on three of the four corners.
To start, you need to make sure that the blade is tilted to 45 degrees as close as possible. I use a Wixey digital angle gauge but, it can be done with a few practice cuts on scrap wood also.
If I wanted to make a box 6" x 10" x 5" tall for example I would start with a board 5" wide and more than 32" long. In fact, I would probably make sure that it was at least 40" long. I then trim one end at a 45, slide it to the right 10", (use a pre set stop) FLIP IT OVER and make another cut @ 45, FLIP IT OVER and trim in the exact same spot at 45 degrees in the opposite direction (thereby creating a little three sided triangle 5" long that is scrap). Now slide the board over 6", FLIP IT OVER and make the 4th cut. (You now have a long and short side and if you match up the miters made with the 2nd and 3rd cuts you will notice that the grain comes as close to matching as anything you have ever done.) Now keep going by flipping the board over and making the 5th cut in the exact same spot as cut #4 (making another scrap triangle), slide it over to your 10" stop and make cut #6 to make the second long side. FLIP IT OVER and cut in the exact same place as cut #6 to make another triangle scrap for cut #7. Slide the board over 6" for your final 45 degree cut on the last short side. If you take the time to mark the pieces as you cut them (example.... A-B, B-C, C-D, D-A , ) and reassemble them accordingly all the corner grain will match except corner D-A but, even that one can come out fairly close.
Anyway, I think that this is a much better way for making boxes with mitered corners even though it is long winded and maybe confusing.
Making splines for the corners is a whole different subject that I could try to explain if you are interested.
Rog