I have a brooder that is similar set up to yours TFred - also outdoors - I cover half of mine with the silver reflectiv insulation stuff and half of the lid is only covered in wire mesh.
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Initially we did struggle with moving the heat lamp around a bit - Where to hang it to get the desired temp? Finally, we gave up and said it was at X height and just cover or uncover more or less of the top to give the desired inside temp. Under the heat lamp is desirable temp and towards outer edges it is 10-15 degrees cooler.
ETA: We (try to) do most of our hatching in January and February so we were looking for insulating value as well.