I bought a set of them over 25 years ago, a diffuser and bounce unit. They have never been used, but I don't know why they wouldn't do the job. My favorite is the Demb Flip-it, but so far, the diffuser that came with my SB-800 has worked fine.
I would recommend the LumiQuest Softbox LQ-925D. It's collapible and take wery little space in your bag as it stores flat. The attatchment to the flashgun is done by velcro or if you dont want to glue the velcro strips to the flash you may just use a rubberband.
I've brought a couple but find that the SB-800's free diffuser is just as good. Shooting directly at people still showed a shadow (had hoped that it would magically disappear using this). Plus you have to blow them up !!!
I have 2 I bought from Photoflex...works fine and does soften the light...I use a piece of paper in front of the l=flash fresnel lens to diffuse the light even more..problem is in weddings I shoot it brings to much attention which is a problem for my style of shooting.