Quick and cheap insulation for skylight
Why not use a sheet of rigid foam insulation? It will block light and heat.

You can cut it to size and, depending on the frame of the skylight, you may be able to wedge it in the frame without attachment devices.

If you need to use something to hold it up, double faced tape on the glass will be fairly easy to remove without permanent damage to the skylight.

To make it attractive, cover in cloth, paint or attach adhesive backed plastic covering.