Because you can't search cults yet I wont be a complete douche when it comes to this...You can easily look down the page and see 100s of threads about this bullshit. Anyway..
So the rail you wont want more than 2 feet. 18 inches max. You will thank me later, especially if you want to use it in the summer. 12 feet is short. You will regret this. Make 2 10 foot rails. Many options with that. For the pvc look here CLICK HERE.
Okay for the rail. You will want to use 2x4s I suppose. I know people that use one 4x4 on the top with several supports along that. It works well and is very stable and you wont risk snapping the 2x4 as i have..woops..Anyway I am not sure how much the wood will run you but just measure it out before you do it. So think about this...4 10 foot long 2x4s...then like a bunch of supports. figure 5 for each rail. All about 16 inches tall. Then you will need supports that make the rail stand up. Dont forget diagonal supports so the rail doesn't tip side to side. I can't imagine it running you too much. Just make sure you get enough but not too too much.
If you dont click the link for the pvc...just drill through the top. Make sure you pre drill with a drill bit that is bigger than the screw. Then Drill through the bottom of the PVC into the wood. becarefull with the pre drilling. Do it slowly so it wont crack. And no you wont catch on them. Also do one screw about every 1.5 feet i think. I am not but 100 miles from my rail right now so i can't go measure them but yeah. Just be smart about it. Not too many but not too little otherwise the pvc wont hold. And i would suggest 2x4s not 4x4s unless you want long screws and metal to support the legs. Just deal with 2x4s...I think if you look in my profile somewhere you can see my rails. Hope that helps
If you have any questions feel free to ask.