*DUMBCAN*Ok the way I see it your biggest problem is getting an even weight distribution, to avoid moments affecting your measurements.
5L capacity is kinda about what you want, gives an internal radius of 10.61cm according to wolfram alpha, round up to 11cm external radius.
I overthought that a little bit, you want a size 5 soccer ball.
Even weight distribution is a bit harder. You can fill the ball with water to make it 5kg, but when you start to take away water in place of air you get a bubble which floats around too much.
The obvious way round this is a two-component system where the density changes based on the composition, but to my knowledge there is no system where we have that much control over the density.
So we need another way.
Possibly... Get a size 6 soccer ball, cut it in half and tape it back together with a measured quantity of duct tape. Weigh the empty ball. Get a lot of really cheap ping pong balls. Using a drill, put small holes in some of them. Hold the holed balls underwater until bubbles stop coming out, and carefully reseal the ball using duct tape. Now you have heavy ping pong balls (filled with water), and light balls still filled with air. By altering the composition of heavy and light balls you place in the soccer ball you can control both the final mass of the soccer ball and the weight distribution. Make sure you use the same length of duct tape each time.
That weigh you should be able to get a ball that weighs between less than 1kg and more than 5kg, and everywhere in-between.
Correction: you can't actually get a size 6 soccer ball. A cheap basketball will do, and is probably built better for the task.
Thank you soo much for taking the time dude!!! This is really helping!!! The ping pong balls are a great idea for better control, i love it! I think ill use a basketball and try things out, probably use the ping pong balls. Thanks man!!!!
cobra_commanderSo, with all the Google in the world you are telling me you can't find a ball that you can put up to 5kg of water (that is 5l dumbass)?
Talk about stupid and unmotivated.
Thank you sir for your formidably useful input.
Chubbs.can you shoot expandable spray foam in there? figure out how long you have to hold it down for each weight
Thats a great idea too, ill look into that, maybe a solidifying spray thing, and maybe ones with different densities, would be great.
STOKED ON THE HELP NS!!!! +k to all who helped!!!