Electronic Contact Cleaner works really well on sticker residue and does NOT discolor plexiglass/lexan like any orange-based ones.
Use either the "GB" brand (mainly black spray can with red and white letters) or "LPS" brand (mainly green with white and black and red). About $4.99 to $11.99 depending on where you find it. I prefer the LPS stuff myself but is usually more expensive and harder to find.
Make sure you get "electronic" not "electric"... Electronic is for circuit board use, Electric is for like cars and big machines.
Also works great to polish game cartridge edge contacts, fix nintendo game unit slots, and loads of other uses. Removing sticker goo is just a side benefit.