These frosted caps should have originally come in the kit, but since Vistek now has them in stock (1 pc @ the Downtown Toronto Store as of 5/15/2010, because I scooped up the other 2. I believe there was 1 in Missisauga as well), I bought 2 of them for my Quadra kit.
Elinchrom calls these multifunction caps because the cap acts as a diffuser and gel holder. They cost me $13.50 CAD each. This is a steal compared to what some companies can charge for simple accessories: http://vistek.ca/store/ProPhotoLightingComponents/112671/pocket-wizard-16-miniphone-to-elinchrom-ringlock-style-sync-cable.aspx
The cap simply snaps on the standard Quadra reflector. With the cap on the head, the tube is fully protected from damage. I can actually literally toss these into my camera bag anywhere without trying to re-purpose the compartments to protect the tubes.
I was a bit curious to see if shooting with the cap on would affect the color temperature.
From what I can see anyways, the bare reflector shot on the right is a bit warmer.