Try Atlantic Towers. They have what they call an "arch in a box". It comes in several different sizes and configurations. Both sides are solid, with a adjustable middle piece. This is a very good product, and I have had one on my boat for 2 years. It holds 3 solar panels
, and has davits
for my 10'6" RIB
I highly recommend these guys. I have no commercial
interest in this company, just a happy customer.
(The picture is pre-solar)