If you can't find any kits avail, you can always install the armor-up kits the army provided us to practice with in the 70's-80's.
Sandbags on the floor and a flak jacket to sit on
.. Definitely fits the 5-ton M39-series and M809-series..
I don't know whether they fit the M939-series..
Sorry I'm being totally unhelpful but I could not resist
.... However, I will keep an eye and ear out !