That's what I did (same as Eunjae's post above). Just cut it out, hole-punch it to fit over the diffuser mount holes, screw diffuser back on. Cheap and simple. Doesn't even matter if it gets crinkled over time - it's not going to be taking too much away from a z240 or other high power strobe.
And of course original credit goes to Alex Mustard's article, at least for my inspiration. Couple these with the new ability in LR (new to me - I haven't used LR very long) to do selective color shifts for the background water tint and I'm much happier about finding a pleasing foreground and background color balance (to my eye at least...I am not nor will I ever be a paid/pro photographer).