P.S. it's also hard to mount the 580EX into the 46" softlighter...it pulls because of it being centered.
No doubt about it. Most of us that are using hot shoe flashes with the Softlighter are either making a bracket or adapter as a custom DIY or from parts readily available from a place like B&H. Everyone seems to do it just a little differently but with some ingenuity and a few neat little components it's not hard to get the flash to lay down along the axis of the umbrella, making is very easy to place inside the centrally located sock.
If you're not terribly concerned about the flash being exactly in the center, Photek does have a version of the Softlighter that is intended for a hot shoe flash and has the sock off center. You can also purchase a replacement front diffusion panel with the off-center sock and that would allow you to use the Softlighter with either strobes or hot shoe flashes.