For what he's doing, 2 6' pieces butted together with a piece of aluminum flashing over the seam on top would suffice
Yup, I've used plastic 5 gallon buckets for splices/repairs in tile/culverts many times...works gooder!
If you're going to do that, wrap a piece of erosion fabric, or roofing or landscaping fabric if you can't get any, tightly around the pipe under the flashing. If you have a little gap between the pipes and high water velocity you could have a blowout and eventually your pipe will washout. Or form a suckhole around the pipe, then when you drive over it will collapse and whatever you're driving is in the hole.
