"I am trying to cut simple miter joints on several boxes I am working on but am having an ongoing battle with slightly open joints. I do a test cut just like you show in your box making video, using a substantial piece of wood and get a wonderful outcome - no light between the test gauge and the wood - but after cutting the box's sides I end up with an open miter. There may be a slight error at each corner which accumulates in one corner when I hold the box together by hand, or there may be one joint that is open.I think there can be some noticeable deflection in a thin kerf blade. I've noticed it myself. Also, I've noticed that if any of the parts are not held tightly enough to the surface of the sled, and they are allowed to rise up slightly during the cut, it can lead to a slightly open miter. Not all woodworkers have the same strength in hands and the pressure from the blade during a miter cut seems to create a bit of lift, rather than just downward pressure that would find in a 90° cut. While I normally keep the blade as low as possible to make the cut through the wood and no more, raising the blade slightly may give it more of a downward force. Try it and see if it improves the fit of the miters.
"I use a miter sled fashioned pretty much like the one you use. One factor that has just occurred to me is that I am now using a thin kerf saw blade. Do you suppose there is some deflection in the blade that introduces a slight bit of error into each cut? In the past I can't remember having this problem - and I am pretty sure I was not using a thin kerf blade at that time. It is a pretty new acquisition. It was recommended by the Freud dealer for box and other small woodwork projects because of the superior, smooth surface left by the blade. It certainly does that well. My thinking is that when you are nibbling off the miter cuts at the end of each box piece there isn't anywhere near an equal pressure or resistance from wood on each side of the blade, perhaps allowing it to deform every so little. Maybe I need to build a new miter sled?"
Also, I assume that you are using a stop block to control the length of your parts. There are two things that normally go wrong with miters. Either one piece is cut to the wrong length, keeping miters from closing, or the angle is off.
I use a thin kerf blade for resawing stock and for making inlay, but for most cuts I use a full thickness blade with 1/8 in. kerf.
I'm not sure if a new sled is necessary, but they are easy enough to make.
Make, fix and create...