And you lot have real short memories.
Before Corflute.
The humble Cardboard Box and Duct Tape.
You gently relocate your Amber refreshment from the box.
Open out the box to a flat sheet and then fit it in place with Tape.
If you didn't like to advertise, you put the printed side inside against the glass.
If You have a Big Green Shed nearby you can get Poly-carbonate/plexiglass/Perspex sheet etc. also.
They'll have suction cups, velcro, Duct tape etc. available for fixings.
Also, any Boat repair place could knock something up. If you don't trust yourself. Be for a Price of course.