How to access piping inside solar pool heater panels?

A customer of mine (I have a home maintenance business) has a solar pool heating system. The system is rather old and so far I haven't been able to determine a manufacturer. There is a slow leak dripping from the lower "header" tube, but I'm not sure the best way to open the panel to find the actual leak. I know enough about how the panels work and have researched different repair methods. The panels have a black material covering the top and corrugated fiberglass underneath. Everything is connected with steel straps and pop rivets. The only way I can think of to get the cover off is to drill out the rivets, but this seems like a very poor design for a plumbed system. Did they truly think there would NEVER be a need to gain access for repairs?

If anyone has experience with these type of panels, can name a possible manufacturer or has any tips on how to go about opening them up to find the leak, I would be very appreciative.

Thanks for reading.