Iv'e built several of these panels for myself and others. These are smaller 20watt models, so only one clamp was needed.
I used clamps for the Magna grills:
MAGMA PRODUCTS | DUAL RAIL Mount 1" | T10580 from the ShipStore.com ™ on-line catalog.
We've just installed panels on the Womboat but I'm not impressed with the rail clamps.
Norstar .. where did those clamps you've used come from ?