First time in Florida during the summer I had to cover that skylight too to stop the heat from coming in! :B
I used 'removable limo tint' from Walmart.
I didn't like how pitch dark covering it in foil was. And how tacky.
The limo tint took down the sun and heat just as well as the foil did.
Make SURE you get the one marked 'removable'. Just cut it to fit.
Get a spray bottle and add water and dawn soap. heavy on the Dawn. Spray the soap/water mixture on the skylight and then place the tint. the more spray of water and soap you put on the easier it is to move around to position it AND it will stay better also. Squeegee, squeegee, squeegee all the bubbles out. The better you squeegee the better it stays in place.
Just a tip. I bought the application kit for the 'permanent' tint because it had a real neat hand held rubber squeegee that works perfectly for applying the tint. Just do NOT use anything else in that kit.
I did my toad windows with this stuff and the big window above the couch in the MH. In fact it's still on 4 years later! Looks just like the profession applied ones only you can take it off with ease when you want.
My air conditioner thanked me for covering the skylight.