1. you might want to add a handle to the weights for easy of carrying.
2. either at lowes or home depot they have torn cement bags for cheap I bought a torn bag for 1.50
3. you might consider a 2 1/2 PVC and made it 6ft long. Check your car as to where you'll be packing them
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You might consider 4 inch pipe and it won't have to be so long. Six feet long is a very inconvenient length to carry around.
BTW, running the calculations for volume; the six foot 2.5" pipe has a 2" i.d., using V=3.14r^2*h=3.14*4*72 gives you 904 cubic inches volume for the six foot pipe.
The 4 inch pipe has a 3.5" i.d.; again volume for a cylinder 3 feet tall is 3.14*3.5^2*36=1385 cubic inches, and going to a 40 inch tall pipe gets you 1539 cubic inches of volume, or more than 70% greater than the volume of the 6 foot tall 2.5 in diameter pipe.
Another efficient method of making your weights is to use the plastic fence posts. They are square so there is more volume for the same width and significantly, they don't roll around.