We're doing an endurance race in our Lemons/Chump CRX that requires a minimum weight of 2000 lbs. We'll need to be able to remove said weight for LeChump. In my old EP Civic, I had some flywheels bolted to the passenger side floor, but I wasn't this far under-weight on that car...and honestly, I was never thrilled with the prospect of one of those things coming loose in the middle of a turn.
Thoughts?
Weld a box to the floor to put them in and bolt them down?
Two passengers? 
We had to do something similar to an aircraft (roughly the same ammout of ballast). We built a rack to hold large steel plates (about 4 of them iirc) and bolted them down with about 6 big bolts.
I was never a fan of the idea, but it seemed to work. Plenty of cat-shots and trap wires, and I'd never seen one come loose. We did have to remove and re-torque the bolts every 28 days though.
Just bolt another engine into the trunk
Voila. Safe, adjustable and removable ballast system.
Weld/bolt a rack in the spare tire well to hold barbell weights. If you use the same diameter tubing as a barbell you could use a standard weight clamp to hold them down. And/or make a strap that bolts down across them.
When I ran circle track we would fill old valve covers with lead, concrete, rebar, etc. and bolt them under the car where we wanted the weight. Keep it as far down low as possible.
Set up mounts under the floor to bolt in a big plate of 1/4-1/2" steel of suitable size. The weight's just as low as it can possibly be and serves double purpose to prevent road debris from coming up through the floor.
44 gallon water tank with a remote plug, then hope for rain during the event. 
Elevator weights can work as well and can generally be found used. Heavy as sin and designed to be bolted together/down.
What series are you ballasting for?
As far as I know, none of them let you ballast, including wrl. Wrl allows ballast but not to change classes.
Moar info needed....
IIRC, a chevy valve cover full of lead is about 50lbs. (I have 4 of Gimpy's in my shop, they are for his CP car) speedway has them cast this way $160 / 50lbs http://www.speedwaymotors.com/50-Lb-Lead-Ballast,2164.html
Another (slightly cheaper) option http://pitstopusa.com/c-131890-chassis-suspension-ballast-lead.html
a 5 gallon bucket of concrete might be about 95ish lbs.
All of these are going to take some restraining, you dont want that much weight coming loose in an accident.
15 gallon tub anchored to the passenger floor (multiple places, large washer backing, weld bolts that pass through bottom of tub to rebar reinforcement to prevent breaking loose) and pour in concrete? Cast in a rebar ring and when you are done with race, remove with engine hoist?
http://www.traditionaloven.com/building/masonry/concrete/convert-gallon-gal-of-concrete-to-pound-lb-of-concrete.html suggests 20lb per gallon of concrete. Add whatever wight of rebar reinforcement and there you are.
Find someone looking to get rid of an old cracked V8 engine block, and bolt that to the trunk floor?
Huge motherberkeleying skidplates under the entire car. Bonus points for being able to run over whatever you want.
Let me ride along? That is good for ~300#
If you're adding ballast, you want it low and in the right place, which probably means you want to bolt it to the bottom of the car at the back. It's even better if you can make the weight functional in some way -- a friend of mine once had an 85 pound rear subframe brace on his D Prepared Miata, for example.
Have some steel plate cut to fit in the rear tire well and or the passenger floor. Stack as needed and use plenty of bolts to fasten. I made a plate that bolts to my passenger seat mounts and have plates that bolt to that because that is where I have to add weight..
Do you have an empty spare tire well or is there a fuel cell in there?
Bumper mounted winches?
Funny, I keep thinking that I need to remove that much ballast.
Have my ex wife ride along with you?
Ten used batteries, connected in parallel. That way you'll save the drag and inertia of an alternator.
Lead plates cut in shape of the floor pan, ryno coat or plasti dip after a test fit for non skid let dry and bolt in. The mas will be low and spread evenly
Straping a crash test dummy to a passenger seat would be a real chump car ballast. Or strap it lay down on the floor..
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