It is possible to adjust the torsion springs that hold the Miata trunk open. There are 3 strength settings depending on which slots the springs are in. Be careful not to pinch fingers when adjusting the spring, but it is not too difficult with a long screwdriver to help apply some twisting leverage. This is a great fix for Miatas with trunk spoilers or luggage racks, which the springs will have a difficult time lifting on the lower settings.
PS: There are options for stronger tension springs if the highest setting is still not enough. I.L Motorsport has one option, and I’ve seen rumors that Mazda installed stronger springs from the factory on some cars that came with spoilers.