LISTEN UP.....real simple design, AS LONG AS  ten foot sections are ok for you.  ours is a flat down flat double barrel 30 ft total, here is everything we built it with:
TOOLS
(1) 8 lb sledge
(1) coping saw
(1) power drill
(1) 13/32'' metal drill bit
(1) handheld power saw (optional)
(1) level (optional)
MATERIALS
(6) 2'' diameter 10' metal conduit casing pipes (2 for each section) - these are about $9 per 10' length at home depot and are about 1/16'' thick - don't get the galvanized steel it's like 3 times more $ and three times thicker
(2 or more) 10' sections of same pipe for vertical supports
(1) 10' length of 3/8'' threaded rod
(24) 3/8'' washers
(12) 3/8'' nuts
step 1:  with coping saw (or handheld power saw) cut support pipes into halves/three pieces, depending on how high you want your rail to be....it helps to cut one end flat, and the other at an angle since you will be driving it into the ground.
step 2: drive support approx. 1' into the ground with 8 lb sledge. (the bigger the people sliding the rail, the deeper you want to go)  hint:  do one support at a time as opposed to all of them at the same time, since they won't want to go in straight anyway, and you can line them up if you go one by one
step 3: since the sledge fucking destroys the top of the pipe, you have to cut it level with the coping saw
step 4: using power drill, put one whole ten foot section of pipe on both sides of the support, drill one hole through all three, making sure you go as close to the top of the support as possible since the top of the support is barely below the surface of the two bars you will be sliding
step 5:  entire assembly is about 6'' wide, so cut the piece of 3/8'' threaded rod to about 8'' or 9'' sections (enough to go through all three bars plus room for washers and nuts).  put rod through all the way through the hole, with washers at each end and where the bars come in contact with each other, and thread 3/8'' nuts around both ends.  tighten like a motherfucker.
step 6: repeat steps 1-5 for each support so you can line it up with your eyes, ours has four total
NOTE:  this design does not allow for long, seamless rails, instead there is a 2'' dropdown at each section, but it's really nothing, and the rail rides really smooth, especially since you can buy all the materials at home depot, and the tools at a pawn shop (if you/your parents don't already own them) all for about $100
hope that helps and good luck
-Strode
Abba Zabba, you my only friend