When I needed to do something similar I milled flats on the corner of the angle in two places, drilled it through the flats, and bolted it down to the table T-slots. Don't crank it down too tight or the angle won't be 90 degrees any more. If you need an accurate angle you'll want to mill the sides of the angle to equal heights and the edges to equal thickness.
Drill the face of the angle in convenient places and use your mill clamps to secure the work to it.