Scott,  I think the best answer is to simply tap those bolt holes for the thread size of your bolts.  This does a number of things.

1.  There is nothing to fall off inside the plenum, no nuts, no bolts or no nut plates
2.  Its a few fractions of an ounce lighter
3.  You never have to worry about getting onto the back side of the plate (looks like you intend to weld it to the plenum tube.
4.  Two of your nuts are pretty close to the wall of your plenum if that bright circle is indeed where it will be placed - can't tell for sure but could be interference with the wall of plenum.
5.  You never have to worry about it again.

Ed