BesseyTools WS-3+2K 90 Degree Angle Clamp for T Joints and Mitered Corners (2-(Pack))

Easy to use, just be sure to set up joints carefully if you want accuracy. A little hefty on the price tag but should last a lifetime if cared for. I have four of these so I am now good to go!

Had an old pair of these for years that were ok New ones are not accurate at 45 degrees and are not level with the old set so I can’t use both sets at the same time. And yes my 45 degree mixers are very very accurate.