What surveyors call swing ties would work better. Dimensions across corners to break it down into triangles. I'm assuming that the 12-1/2" and 11-3/4" sides are perpendicular and that the 3-3/4" side is parallel to the 11-3/4" side. Dimensions as shown would nail it down.

Thanks for thinking of other 'bolters, John.
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Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.