Steve I am also a big fan of Flirc cases and I have two Pi’s in them now.
This was the document I was thinking of but it does not mention USB WFi dongles.
Based on what you wrote about the wireless fluctuating in the Pi it will be a struggle.
There is a thread here that I am having a read though
Have a look at the link in this post to Amazon Germany
I have never tried these Steve, but sounds positive and cheap