Dick Hanscom should perhaps approach whoever is producing the current crop of “replica’ banknotes being offered on eBay.

I have just seen the replica £20 George VI Fiji specimen. The Fijian head watermark in the note’s window is reproduced in disconcerting detail – quite sufficient to fool most observers. My attempt to reproduce it and here does not do justice to the skilled artisan responsible for the reproduction.

I can suggest a couple of approaches. First, since you showed a photo of the paper apparently without backlight, you can see that the particular watermark Dick wants to use is very prominent. It is visible with the note just sitting on a tabletop. It can be enhanced by placing it on black paper. And, since that particular paper is not scarce, if he's interested in spoiling one piece by soaking it in vegetable oil, I suspect it would be even more legible after placing it on the same black paper. When he's done with the exhibit, it's possible that the paper could be essentially restored by treating it with a degreaser. After all, it has survived five decades of submersion already.