Also see my keyboards: https://imgur.com/a/KxBSR

TaoBao Adapter, $20: https://world.taobao.com/item/542300386181.htm +$3 wakeup touch module https://world.taobao.com/item/548629862732.htm

Handheld SCI Bluetooth Keyboard Adapter, $39:
http://handheldsci.com/kb

USB2BT Adapter (PIC-32 based), $60: https://www.amazon.com/dp/B00PI7ZDOI/ Schematics (japanese): http://sohta02.web.fc2.com/usb2bt.html Unboxing: https://www.youtube.com/watch?v=SRSyqky-WWk

DIY HID Relay (Arduino Mini + USB Host Shield, MAX3421E based): https://hackaday.com/2016/09/04/convert-any-usb-keyboard-to-bluetooth/

The infamous GH60 thread: https://geekhack.org/index.php?topic=40514

It works!

Was: HC-06 module ($2.48)
Turned into RN-42 bluetooth HID module ($25-$50)

Had to fetch 10 year old PC with an LPT port from the closet. So yeah, it was pretty troubling (and dusty).

RN-42 firmware is here:
http://pastebin.com/V91PZBnJ

Blueflash software is here:
https://mega.nz/#!UxBFnICD!v1tmeMLrGa3JOClGI7XTUgPNiypeGWVVAcsaCVDCo04

Arduino keyboard test code is here:
http://pastebin.com/S5Z4PXay

Also pretty nice pics are here (breadboard, shields, and all that shit):
http://elasticsheep.com/2012/06/bluetooth-module-parallel-spi-interface

Also, apparently it's possible without LPT/Windows XP, with FTDI adapter (I haven't tried it yet):
https://github.com/lorf/csr-spi-ftdi

Also, this manual can be useful (uses FTDI as well):
https://mitxela.com/projects/bluetooth_hid_gamepad

Also, this video:
https://www.youtube.com/watch?v=fWXJDNcbZAA