Just a piece of advice to consider...look up "rolling ladder" on craig's list. You can often times get one that is 6' tall for under 150 bucks. I've seen them go for $50 here in Indianapolis.
These make ideal towers to drop in from, then all you have to do is attach the ramp that you have built on the other side. Once I get mine finished, I'll post pics.
Also, consider using PVC pipe pieces spaced about 6 inches apart running perpendicular to your drop in ramp. It is very fast and durable. I used to use the nylon latice, but it broke easily once the temperature dropped into the teens.
I'm able to get enough speed from mine to hit a 2 foot kicker.