On my previous boat I made simple screens by cutting a rectangular shaped pieced of fine mesh mosquito netting big enough to cover the companionway hatch. Then I stitched 3/8" nylon line all around rthe perimeter of it. When I laid it over the hatch, the weight of the nylon line held it in place.
I also made similar screens for the Lewmar hatches. They were in the shape of 3 dimensional triangles, with mesh covering the open part of the hatch, and the main body of the device was made of rip stop nylon. They also had 3/8" line around the base, to hold them in place. Those allowed me to leave all the hatches open for ventilation in a noseeum area.