I had the same issue, I purchased two 1/4 x 29 1/2 inch hex head bolts and lock nuts. Inserted the bolts from the rear facing out by carefully stretching the mount ears. Installed the plate and held the bolts with an open end 7/16 wrench slid behind the plate.
2015 Wildwood 27RKSS
2015 Silverado 2500HD Duramax
These types of brackets are meant to have the bolts come from behind and be permanently in-place. You may have to loosen the allen heads, put the bolts in, and re-seal the frame. the last I installed, I was able to get short license plate bolts and tweak the wings out far enough to slip them in place.