cheap, effective whole window shade

ask at a window shade place. they make shades that pull up/down with metal foils in them, and blackout curtains. they leak a bit around the edges, unless you can recess them a lot.

for skylights, they make shades for them too. motorized ones if out of reach. you could cheap it with form fit cut pieces of styrofoam.

installing window shade film to double pane windows would invalidate any warrantee, so i would bet al foil would also.

I'm guessing any reflective material is going to increase the temp between the panes -- thus the warranty issue. The tinting really needs to go on the outside pane. Shades that reflect would seem to have the same effect -- but probably with the airgap, it's not as damaging -- and not very effective at reducing radiant heat either. Styrofoam in the skylight opening probably has the same "overheat" affect.

tinting is never put on the outside of windows. it's ok for single pane installs.

my skylights are plastic, and there's quite a distance from the skylight to the bottom of the tunnel so the heating problem doesn't occur.

if you have glass double pane skylights, i'd look into a commercially made blind. you can also get skylights that have a blind between the glass panes.