For the 430 EX the Radox 300ml Clean and protect handwash is a perfect fit. Straight cut across 2.5 inches from the bottom and it is an easy but snug fit. It is slightly clearer than the Carex plastic but have just tested it - I must have the cleanest hands in Christchurch

- and it still works quite well. All a bit subjective but I have a little comparison test I do which is to shoot something in a recessed corner of the room. If the diffuser is contributing little fill light then the shadows in the overhang from the bounce on the ceiling will be pronounced and clear. The less intense the shadow (whilst maintaining the overall lighting) the better the diffuser is working. A quick bounce comparison of with and without the Radox showed that even though this is relatively clear it was still pushing some light forward.
So, Carex for the 580 and Metz 54 and Radox for the 430. I have not got a price on the Radox one as I helped myself (in the pursuit of results and research) whilst my other half was out. I'm sure I'll be told in clearly understood saxon english exactly how much it costs when she finds the pudding dish full of soap next to the sink.
John