This is a place where you can find or modify a bucking bar to fit. I would not buck those rivets freehand but instead would clamp the bucking bar in a vice, put the work in place over the bar, and drive it normally.
However, think about the context in service. These are used to hold nutplates in place, and mainly against rotation. Screws will be inserted through a lock striker plate, at which point the rivets are both invisible and backed up by the clamping action of the screw/nutplate combination. Also, the channel is fairly thick stock, so it will function just fine if you 1) countersink the channel instead of dimpling both channel and nutplates, and 2) use 3/16" pulled flush rivets there.