I recommend reversing the box joints in that plan. The frame rest flange is unsupported at the ends and it is weak and prone to breaking off when prying frames out.
This is what you want:
If you start making joints (3/4" or 1" joints both work) from the bottoms and don't cut the top of the end boards it will work out just right. Start the end boards at the bottom with a notch and don't cut notch at the top where the frame rest is. On the side boards start at the bottom by leaving a finger. You'll end up with a 5/8" wide finger at the top of side board and you clip that back to 3/8" long and it fills the frame rest rabbet and supports the ends of the frame rest flange.
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