Thanks all for the discussion!
I taped down a couple of straps to my bottom seat cushion, just to keep them from twisting. I took the rear seat cushion out. This let me keep the straps from getting twisted while I placed the ballast.
I put each of three 50 lb sand bags in their own trash bag, for mess patrol. I used a big reusable lawn and leaf bag to contain all the bags. I had a cam cleat strap go around the longitudinal axis, and a ratchet strap go around the roll axis. That was what I had in my collection. Here, the cam cleat strap excess is being fed through the hole in the top seat center so I can wrap it around the box beam and tie it off.
The red ratchet strap that goes around the roll axis also has excess strap. I fed that around the box beam on the outside and tied it off.
Besides the straps, I used the seatbelts of course. The bag had a couple of straps sewn into it and I went through those straps to keep the seatbelts located.
Seems secure. I'll find out. Any feedback is welcome.