Vivitar Ultra Wide and Slim / red filter mod version 3.
Following on from v2 (red filter attached by means of electrical tape), and v1 (red filter held over lens by hand)...
...come this, my latest refinement - a £1.50 67-52mm step down ring superglued in place. Now I can screw the red filter on, stick a lens cap on that, and chuck it in my bag - no more bits of tape everywhere.
(You can see where I initially stuck it, then realised with the extra depth of the step down ring I'd get really obvious vignetting where it was, so I prized it off and repositioned - having to slightly overlap the viewfinder so that the filter sits central to the lens)