I want to have a go at making a baby flounder fly, something like this one.

 just use some of the stuff in the tubes...choose the time frame so you have enough working time.

Thinking out aloud...I would take a little different approach maybe.
What if make a basic mould from a actual little flounder...say out of automotive bog...
 Then use silicone rubber or something along those lines... vaguely rem an old thread on making soft bait lures.. think it was oldfart used to make some good ones...can use dyes to colour.
If still wanting to use feathers these could be in bedded in the edges/ tail??