Dave,
You've received some good advice in this thread. It seems that you already have a Photoflex to Elinchrom speedring, and if so, the Photoflex Octodome makes good sense.
If you didn't already have the speedring, I think the 53" Octa Midi from Elinchrom is hard to pass up. $308 from B&H, and the light that has been coming out of mine is nothing short of wonderful! The only drawback that I can see (compared to the Photoflex offering) is that it doesn't take grids out of the box, but it could easily be adapted to do so with a little velcro and free time (I haven't had the time or need to do that yet). And, like all Rotalux softboxes, it folds up like an umbrella and comes with a travel case for transport.
Best of all - the thing would fit perfectly on your new RX600! 
So... if you already have a Photoflex to Elinchrom speedring, you might as well save yourself the $30 and get the Octodome. I can't personally vouch for the Octodome, but it seems that most people here love them, and that Photoflex make some really nice light modifiers. What I can do is vouch for the quality of the Octa Midi, and I can promise you that you will not be disappointed if you decide to go with the Elinchrom offering.