I can get my fingers down behind the corner of the barrels just enough to hold washer and long nut if need be, but depends on your fingers...?
On mines' last incarnation, due to unavailability of the long bolt and the internet, I used a Socket cap bolt long enough (~6"+), and with one or two locknuts on it. Was easy enough to get in, but with the internet I rapidly found a stainless Long nut from Lightning Spares- seems easy enough...?