For my build, I used cheap carriage bolts that I buy by the pound at a local home center. Every one of the 5/16" carriage bolts fit just fine into every one of my 8020 series 15 extrusions.
You prompted me to make some measurements. The slot size in a piece of 8020 scrap measured .321. The square neck of several carriage bolts measured between .308 and .312.
Just remembered, I bought some of my carriage bolts from McMaster and they worked just fine too.