This may apply to a limited demographic, but I figure it's worth posting should anyone be faced with a similar situation. The setup is this. Pick a lens, any lens. Throw a "slim" filter on there that has front threads. You'll find that the regular Canon lens caps don't quite go in all the way anymore (on regular filters, this does not apply).
There's also the fact that Canon caps can be a bit of a pain to put on and take off with a hood attached, since you have to squeeze them around the perimeter. Many replace them with "Tamron" or "inner-pinch" style caps.
Here's the success story. I picked up some cheap Chinese Tamron cap knock-offs in 77mm size to fit my Heliopan SH-PMC Slim UV filters on the L lenses below. They fit like a charm! They go in all the way, plus have outside and inside "pinch buttons" for removal. Granted, they're not quite Tamron quality - but work. You'll notice this on the button mechanism. I only had a 72mm Tamron cap to test and they work just as well (goes down all the way). IMO, I'd spend the extra money on the Tamron caps if you don't mind the logo. Next time, that's what I'll do.
Since I hate having the caps dangling around, I used an X-Acto knife and quickly removed the little plastic loops and the retaining loops attached to same. You may or may not wish to retain this feature.
