The stock Grado headband / gimbals will fit with precision on those cups. The person who Cabillas used to cut the cups, did so with great precision. I never tried the G-cushion on the Magnum. I really like them on the PS-500 though. So, you bought the G-cush (the large bowls?). They are quite comfortable in my time that I've spent with them.
I placed an order earlier for the G-Cush and the L-Cush knock-offs that are made by Zonk and sold via Amazon a bit earlier today. I know some people have responded that they have had some interesting experiences with them from fit issues to other possible issues. But, I thought I'd take the plunge as it would be interesting to find a pair of the L-Cush variety with a slight be of more comfort.
I always wanted to try the G-cushion before but their price is a little number in Canada (65$ + tx at the local audio shop). So when I found a deal for a pair that had been used for a few hours only for cheap, I jumped on them.
I may end up going back to L-cushion but at least I will have tried the bagels :-)