Need ideas for a flywheel-like part

I need a flywheel-like part, and was wondering whether anyone had
suggestions on what kind of thing I should be looking for.  I've
looked briefly through the McMaster-Carr and Stock Drive Products
websites, but being a software guy I'm not really familiar with the
hardware they describe. So I'm not sure quite what to buy.

The project that I'm working on involves making very accurate
measurements of the velocity and acceleration of a DC motor using an
encoder and microcontroller. I plan on using the data to help me
develop a model for the dynamics of a small mobile robot. There are a
lot of steps before I get there, but right now I need to be able to
attach a wheel with a known moment of inertia to the motor and then
measure how fast it spins up. This procedure will let me compute both
the stall torque for the motor plus the torque/speed gradient. I've
already been able to match the manufacture's specification on a bare
motor. If I can mount a wheel with a non-trivial moment of inertial on
the shaft, I should be able to directly measure the effects of energy
lost through friction in the gear chain.  I don't need something
really massive, maybe just the weight of a typical cell phone or
paperback book (though I could go heavier).

My test motors have a 6mm shaft about one and a half centimeters long.
I'd like to be able to attach and remove the test wheel easily.
Ideally, I'd like to get a couple of different wheels with different
moments of inertia. Also, if I can't get the moment of inertia from
manufacture's specs, then the shape needs to be clean enough that I
can take measurements and estimate a value. And, just like everyone
else, I need to keep cost within limits of my meager hobbiest's budget.

Thanks in advance for your help.

Gary