Some more detail would be helpful in suggesting the best answer for your aircraft, there is wide variation among types.
I have flown with many types of ballast. Most commonly, I use the cast iron 50 pound weights, as we have 20 of them. Their downside is that they must be very well secured, as they are a very real occupant hazard if they get loose. When I had 700 pounds of them in the back of a Cessna 206, I heard a mighty thump during the recovery to positive G during a messy spin recovery. No harm done, but they had not been tightly enough secured.
Lead shot in 25 pound bags is good, and less a hazard, but they lack a "handle" and can worm their way loose. Bursting one is a mess.
I used two skids of borrowed bottled water a once, as I had to have 4000 pounds. That takes lots of arranging.
Many times, I have used cloth bags, which I filled with sand or gravel locally. This was very convenient in the bush, where more formal ballast is not available. With floatplanes in the bush, I have also poured water into the closest float compartment. I got strange looks at the dock, as you're really supposed to be pumping it out, but it works.
Placing ballast on a seat should be a last resort choice. It can be hard on the seat's structure, as you should be securing it more tightly than you would a passenger. The aircraft flight manual may give some guidance as to the preferred method of securing "cargo". You may also find that special fittings are necessary.
Pay very real attention to getting the C of G location correct. A number of times I have been presented with a ballasted aircraft to test fly. After the first mistake I found (where a Cessna 185 was errantly loaded 2 inches behind the aft limit), I got used to checking the loading myself, and certainly found errors from time to time. Prepare a correct loadsheet, and satisfy yourself that the pilot actually has reviewed and agreed to it. If you're the loader and pilot, have someone else check what you have done before you fly!
Whoever is flying the plane will (or should!) take a very real interest in the effective security of the ballast, particularly if it behind them. The 9 G certification requirement for security should be a minimum, though you might reach the capacity of the cabin hardpoints for attaching tiedowns.