Edit- NVM, I just noticed you're looking for heat gain
I've been thinking about doing something similar and have been hung up about whether to do external roll up shades, like these...
Shade Cloth DIY Patio-Enclosures
Or the external window tint like this...
Gila 3 ft. x 100 ft. Titanium Heat Control Window Film-10363486 at The Home Depot
Either way though, both solutions look feasible.