I did notice your question in t'other thread... I just needed a bit to think of some ideas to help you get those creative juices flowing.
I did some searching online and I did come up with the following possibilities. Of course, there aren't any pre-made "creature from the Black Lagoon" (CftBL) costumes (that I could find anyway)... so some of this might be a stretch.
First, I read somewhere that for the "costume portion", you could get a pair of sweats or spandex and just attach some fake foliage.... but that alone, I thought, wouldn't look "super-cool". So I looked up some photos of the CftBL... and decided that would look better as an accessory... because CftBL looks more armoured or scaled... almost like a dragon.
So... I was thinking what about breastplates and belts? Something like this has a CftBL "feel" to it.:
Breastplate Example
Dragon Belt
Both of the above look to be English sites... with prices in pounds.... so you'd have to do some searching... but something like that in conjunction with a green "suit" (either store bought or made by your wife based on a pattern)... fake leaves... and the mask, hands, and feet... would probably work.
... but that's all I have for you. Maybe you or someone else will be able to build on this and come up with a better idea. *shrug*
Best of luck!