I just lowered my chair until my belly was under the desk and moved the monitor and keyboard forward. Probably not the best of OWHS but worked for me until 38 weeks. I did find that bubs would kick the desk wherever it contacted with her, she wasn't impressed with the squished feeling.
I agree that you might want to check out those fit balls. Will keep your back straight as well as helping with keeping your pelvic floor muscles working. When they are on the bases then they are quite steady. I was also recommended to use one (which I did at home) to get bubs into a better position (possible breech at one point) and keep her there.
Good luck with the rest of your pg