I was in your same boat 3 months ago. I had the cart loaded at Home Depot....there is no way you can make one cost effectively with new parts. I bought this one from agrisupply http://www.agrisupply.com/string-wing-5-...campaign=SSIDE/
It works great on my ATV. You can also hook it up to a tank sprayer and pressurize it, although it makes the roundup go a long way.
Also, look online for a free shipping or discount coupon for agrisupply. They shipped in like 3 days