I've read that custom-fit products for the older 550EX can apparently be used on the 600EX, although the fit is snug.
I have searched for the same and found low profiles ones
I admit I've bought a non-low-profile original sto-fen for a ridiculously high price because the low-profile versions looked as if they'd fall off in the lightest wind breeze or camera shake - and I didn't find any comparisons "large" sto-fen vs. the smaller rip-offs.
only issue issue is that they do not stack when not in use as the ones for my 580 EX do. Unfortunately, I have not found a perfect solution for 600EX gel filters and have now resorted to having to make my own ordering large sheets from B&H an velcro off ebay.
That's my problem, too - can you describe (or take a pic of) your home-brew gel holder version?