Here are the bits you get with the M1025 HMMWV from QRF.
Here are the assembled views. I'll apologise for the big blob of BluTack holding the Mk19 on, but I couldn't keep it in place any other way. It should glue on a fair bit nicer. There's a little depression on the roof (if you zoom the photo above) to place the upright on the weapon mount into.