I saw a version similar to this in a video posted on Matt Kenney's instagram account.
I am running out of room in the house for major furniture builds so I started to think about making small jewelry boxes or keepsake boxes, maybe to sell. I wanted a sled that was designed for nothing but cutting 45's on box pieces. What I ended up with is a two piece jig. One part sits in the left miter slot and catches the off cuts, the other part is the sled itself that holds the work and slides on the right miter slot.
The sled is 10 inches by 24 inches and has a tee slot for a stop block. I have a safety box on the back edge to keep the blade from being exposed on the back side of the fence and there is also a piece of plexiglass 2 inches wide over the blade end that allows me to see my work but keeps my fingers away from the blade.
I built some practice boxes to test the concept and ended up adding a toggle clamp to the stop block to make it much easier to hold the smaller pieces.
I should make a comment about safety - In the first three picture I have the blade a lot more exposed then I ever would during operation. I had it up to make sure I had all my safety clearances covered properly. I didn't noticed that I had left it that way while taking the pictures until just now.
Four pictures of the sled and the last is one of my "test run" boxes, not even glued, just tape.