Does anyone know what sort of handle would fit this main valve? Prefer to avoid using the tongue and groove pliers again!
You can get a replacement handle for this situation which bites into the stem. This type of replacement handle is fine if the valve is not regularly operated and if it is not excessively hard to turn.
Not the answer you want but the only way a handle will fit on that chewed up stem is if the stem is replaced.
Anything that the handle used to engage with the stem is gone, whether it was square shoulders, knurled, fluted or flat spot. I do not see anything for the handle to engage to turn the shaft.