I might roll your brake plate about 90 degrees clockwise. This will hide the anchor point on the plate to behind the lower frame tube.
A short strap from this anchor point to a point an inch or two down on the inside lower frame tube. This will keep the plate stable and allow for wheel adjustment. When you mount the brake arm on the plate, have it point a degree or two back from straight down. This will give max movement of the arm.
Thats about how I would do it. If I ever do a hardtail. I doubt I'll build a hardtail, I like my back too much.