Metric Dowel Nuts are what you're looking for.
https://www.google.com/search?q=metr....0.lJ4hbf3d99w Not sure what size allen head bolt that is but it looks like you need one with a small outside diameter washer which are hard to find but may be the same size as what are on many handlebar stem clamp bolts. LBS may have them (bolt w/washer) or order from Ebay, etc.