I am using a Koss CS-100 Computer headset on my BITX40 with excellent results. These cost less than $US20.
I have tried a couple of other inexpensive Computer Headsets and have found that with some the microphone is
not sensitive enough for use with the BITX (i.e. output is very low).
I have also used a more expensive Yahama CM-500 that works very well.
The nice thing about computer headsets is that the non-USB models normally use 3.5mm stereo plugs for both Headphone and
mic connections, making it easy to get jacks and to drill appropriate sized holes in your enclosure.
P.S. For PTT I am using large momentary switch mounted in a plastic pill box with a 3.5mm stereo jack on the bottom. This allows me to
use a normal 3.5mm audio cable to connect it to the rig.