No, building one is the worst way to learn. When we go to COP at the beginning of the year we'll teach you how to slide a box. You'll build one, and then it'll be too easy in a week and then you'll get bored.
acctually I wouldn't. Your own backyard is a great place to learn. I pretty much learnt how to slide rails on my 10 foot 2 bar PVC pipe rail in my backyard
But when was the last time you used it? Boxes are a waste of time in your back yard, rails are another story. I think adam should learn how to slide boxes at COP and when he gets better he can build a rail and use the bikerack.