A very simple camoflage net canopy. Would change this into a great exhibit. Obviously also ignoring the water moat.
5 poles in the pattern of 5 on a dice. With electric barriers. It surely would also be at a reasonable price and possible to use in house staff to construct.
It would be amazing to see a zoo like Lisbon make a stand and implemet this change. Then to educate the public as to why they have the canopy.
It will be a very wise zoo that makes a stand on such a matter!!
Its not as completely open as some enclosures and is very roomy. But a shade cover as you suggest would be a huge improvement. I'll bet at present the Gorillas spend most of their time just where they are in the picture- under the covered area...