Expanding nylon dowel to fix a U shaped rail fails
Can you slip a small screwdriver through the slot to hold the expansion piece into place until the screw expands it enough to stay in place on its own?

Otherwise, maybe you can find a small metal spacer to slide on the screw behind the bracket to help hold the expansion piece while you tighten the screw:



(image from TwistedThrottle.com)

A stack of small washers might also work.