@Aroth, I know I already sold you the usb one, but I just remembered this post:
https://www.mobileread.com/forums/sh...ad.php?t=71367
Which links to
http://darron.typepad.com/not-nearly...dx-part-1.html
This guy put a bluetooth serial port in his kindle. I don't know if it is thin enough to fit in that empty space