I was going to say the same thing. Glue the washer to the inside of the beam before riveting. You can even use plain old silicone caulk. The glue just needs to hold the washer in position temporarily while you install the rivet. Also, if you can remove the beam from the boat and flip it so the holes are pointing downward, it will help a lot. Gravity will be your best friend or your worst enemy in this case.

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