A Swing Bridge instead of a Duck Under

[ngauger]

Then drill it & attach it to the other supports. It should be attached to the boards at the 15" marks. This should give you a perfect 45, 45, 90 triangle. it should also "Level out" the horizontal 2 X 4.

If you want to put a facade on the front as I did, get a piece of 1/4" plywood or other wood & lay the frame on top of this new triangle and mark the cuts so you can fit it on the triangle - to make it look nice. (this step is optional)

The board is attached to the front, but here's a rear view - showing the plywood through the frame.