Call around to steel suppliers and see if they'll give you a piece of DOM tubing
here's one place I found that may have something- they may even have 4130 but you really don't need 4130
http://www.nitromfg.com/
tell them it's for a school project- they might have a piece in the Cut off bin.
I could send you a piece of tubing but it will be about 5-7 bux to mail it.
You could even use regular Mild steel tubing since it is not carrying or supporting any weight- just stopping the Down tube with a slit from compressing.