Softboxes for portable flashes?

[J_S]

I shoot photos for a BMX magazine, I shoot on 35mm slide film and bw negs. I use a Canon 540ez flash for my main light and a Canon 420ez as a fill light for low ambient light scenes or as a kicker (bright light from behind) in daytime (high ambient light) scenes. I use Pocket Wizard radio slaves with these flashes and this setup works well for me.

Since reading a few portrait books, I've become very interested in taking some nice B&W portraits. I've tried in the past with my flashes I use for my BMX photos but the bare flash is a way too smaller light source, creating some harsh shadows. I want the medium sized softbox look like here:

Portrait link

What I want is a softbox for my Canon 540ez flash to get the nice gradiated light, and to continue using my 420 as a kicker light (Like the one in the example photo above, on the left side). Are there any softboxes available to fit portable flashes? I'm looking for something that will fold up and fit in my bag so I can take it to locations easily, something like the (Lastolite Ezybox ). This product looks ideal for me but I don't think it will fit on my 540 flash. Any ideas?!?!

I DON'T want to get any studio flashes, my camera bag is heavy and bulky enough already!

 

[J_S]

I've just seen the Lumiquest Softbox which is designed to fit portable flashes like mine but I want something bigger.. something like 60x60cm (24" x 24") would be good - so I can take photos like the example link I posted above. I'm not worried about the softbox covering the flash sensor as I use my flashes in manual not ttl mode.