You might be able to jam a sheet of insulation in-between purlins or hold insulation up with 2by2 batterns don't think insulation is that cheap at the moment though but it will pay too go 50mm thick or thicker and the wild might blow it down and I think if you leave it up there to long the birds will have a go at it pecking at it.Any one covered up sky lights before?
Going to change them to fire cement but need a short term fix
Aerosol can of hammerite black