I've got a DJI Spark that I sometimes use for close-proximity flying such as inspecting the roof of a house. The stock propeller guards protect against flying laterally into an obstacle, and the bottom proximity sensor and general vehicle geometry make it unlikely that I'll descend into something, but there's nothing protecting the top.
I'd like to build propeller guards to protect against ascending into tree branches or similar hazards. How much air-intake area do I need to leave, and does the shape of the gap matter? (For example, are mosquito netting on a frame and a rigid open grid equivalent if they've got the same cross-sectional area?)