What is the cheapest and easiest way to cover a skylight?

Window cling film, bubble wrap, or tension rods and fabric of your choice, in which you've made two hems, for the rods.

I would cover something custom cut like foam core board or Styrofoam wall insulation with fabric and use double sided tape to attach. I did this to windows when bedroom shades were light filtering.

Drywall.

DECORATIVE contact paper over the glass for that pretty accent OR to completely cover it, try poster board (it's thin & can blend edges in better) & tape, then paint it.

It is unlikely that contact paper will stay affixed to the skylight window. The differences in temperature between the inside and outside glass, and moisture condensation will prevent it from staying adhered.

You could hang a plank ceiling over the whole area and just cover the skylight. Here's a cedar plank ceiling - https://www.charlestoncrafted.com/cedar-ceiling/

This might help. https://www.finehomebuilding.com/forum/skylight-way-too-hot-best-cheap-fix