Folk seem to have followed my lead in using a Summicron hood for the Summilux 35mm pre-asph (though I'm sure I'm not the first to think of this), so I thought I'd post a few pics of my set-up (excuse poor camera-phone quality).
I've made the following mods to the hood:
• Hole cut to stop hood corner from intruding into the viewfinder (as Leica does on some hoods)
• Four small square pieces of plastic glued inside to stop 39mm filters falling out
• Lens cap held on with a necklace catch (those rubber caps drop off just by looking at them - and the price is ridiculous!) - easily removed and put in pocket for prolonged shooting
• Rear clip-on mount hacksawed off and a 42mm clamp-on mount (reasonably common on eBay) glued on with epoxy - doesn't take long to do, stops the rotation problem entirely, and allows better access to the aperture ring.
As has been said, this hood is a huge improvement for taming flare compared with the original round vented hood - which is totally useless, and even worse when used with filters.
And not only is the Summicron hood more efficient, but the hood and the Summiccron pre-asph is an amazingly compact combination, esp. considering that it's an f1.4 lens.
Last but not least, I don't have to buy girlie hair elastics! 😛