The easiest fix is usually not modifying the cover, but using a rain cover designed with hand access and room for extending zoom lenses.

From the suggestions given, look for a cover with:

• hand-access sleeves or glove-style access so you can reach the zoom ring
• a flexible front section that can expand as the lens extends
• a closure system such as Velcro rather than a tight drawcord at the lens front

Examples mentioned were LensCoat RainCoat and Think Tank Hydrophobia, both of which are designed to let you reach the controls and zoom ring more easily. If you need more complete waterproofing, a flexible housing like the UWA Marine type was also suggested; these can allow zoom control through an external glove-like section.

A tight cord at the front of a basic cover tends to fight against extending zooms, so gluing a filter to the cover is not an ideal solution. In general, a purpose-built rain cover with access ports or sleeves is the most practical approach.