L.D., the best way to weight those is with 10-12 people! Unfortunately I don't often have that many to go around so what I have done in the past is this:
stock locker bag full
250lb bag under rear drivers side seats
250lb bag beside engine compartment (i know this doesn't make it even front-back but it balances side-side)
put a 500lb bag on the floor in the middle of the "wrap-around" rear seating and fill it as full as you can.
if you get enough in the middle bag, put another bag on top of it across the very back of the seating area.
Like I said, I'm going to look into all options including empty battery cases filled with sand, rubbermaid tubs of concrete, etc.