The Miata trunk lid is heavy steel, and the stock hinge spring bars are not up to the task of keeping it open, especially on an incline.
The Cobalt gas strut kit from Moss Miata (900-230) is an upgrade, because at least the trunk stays open now. However, it stops my trunk from opening all the way because the strut is not long enough.
I might reverse this mod at some point, it’s annoying not being able to open the trunk all the way.
**June 2022 Update**
I eventually learned that the OEM torsion bar springs are adjustable, and I was disappointed that the gas strut did not allow the trunk to open fully. I tried slotting the bolt holes on the attachment bracket so the gas strut could mount further up, but that still did not allow for full travel.