Rob asked me to share his Canon EF 8-15mm f/4L USM Fisheye Lens Cap modifcation with you:
" ... I modified the lens cap of the 8-15 4.0L fisheye lens with two small pieces of Velcro (only the soft part). The result is very satisfying! The lens cap won’t fall of anymore even when one presses one or both clips." (thanks Rob)
Rob modified his original Canon EF 15mm f/2.8 Fisheye Lens cap in a similar way - using much more velcro for that mod.