hey whats up...i think that this is a great idea..and i definitely wish i had a backyard with a hill in the backyard.
...but anyway...i too did the same as you...I have looked everywhere to buy rails or boxes to jib on. After finding that so many of them are really expensive, I decided to just make one.
I basically couldnt find online how to make a snowboard box, but was able to find many instructions on how to make a skateboard fun box. So that is what I did, I modelled it after those designs and went with it. I changed the dimensions to fit what I wanted, which was eventually a 1 and a half foot box that is 8 feet long.
I went down to a HomeDepot to get all that I needed, they even cut it for me! So basically all the materials i needed were 2x4's to make the frame and plywood to make the sidewalls and top sheet. Of course needed some nails too.

The biggest trick was finding a material that was similar to "puckboard" (used on most park obstacles at your local mountain).
The closest places to get the stuff were all the way in Canada...so that was a "no-go" for me.
Anyway i decided to go to a TAP Plastics near my house and have them cut me a piece of POLYCARBONATE plastic. They cut it to the size that I would need to cover the top of my box. To cut the cost I ordered (2) 24"x48" polycarbonate sheets (1/8 or 1/4 of an inch thick..i cant remember) to run the length of my box.
To get the Polycarbonate to stick to the top of my box I had to make "counter sunk" holes in the plastic.(there is a special drill bit for that). This allows for the wood screws to sink below the surface of the plastic so you dont get caught on them.
I wish I was able to acquire "puckboard" or "arena board" but I couldnt find any close to me.
As far as the finishing touches on this box, I would like to add coping, but I just havent figured out how to do it...maybe when I make another one (which was the original plan).

All in all it cost me $150 to build the box. With a couple of buddies or brothers you can split the costs and then get to throwing down. I will try to get some pictures up to show you my finished product.
Good luck!

Hey Saul. You've struck a nerve here with me. The slope in my yard played a big role in my growing up as a snowboarder. Placing "junk" on it to jib is lots of fun, and easy, but taking it to the next step is a huge leap in expenses. Snow Park Technologies makes great rails and funboxes, but they do come at a price. There are a handful of other rail builders out there as well, but they get top dollar for their creations when selling to resorts around the country.

Getting into making boxes by yourself can be easy, but it depends on what you want to do with them. Making a wooden-frame box covered with polyethylene is easy. Just make sure to counter-sink the screws through the sheet of polyethylene. This type of box is good for presses and such, but if you want to get sideways on it, you're going to have to really round your edges. Polyethylene is tough stuff, but sharp edges will still grab into it. If you aren't looking forward to ruining a board, you can make a box with steel coping on it. This gets more difficult, because the polyethylene and steel edges of the box have to be at the exact same level, or a lip will form. I'm sure that you've hit a box that wasn't properly constructed. It can get ugly.

As for rails, I used to go to junkyards and pull out already made sort of rails. I used to have this one that looked like the headboard off of a bed. It was single-barrel, and about 3.5 feet tall. It already had a flange on the bottom of each foot, so all I had to do was put a few bolts through some scrap 2x6 boards to make some stable feet. It wasn't the longest rail, but it was good to learn tricks on, because it was short.

I personally haven't gone through the whole process of making a real park rail, but if you have a cutoff saw and some welding experience, it's not all that difficult. Make sure that you plan everything out, and draw it out on paper before you start welding, cutting, or even shopping for the steel. It's expensive, so be sure that you get what you need, and don't have too much scrap left over. Also, don't use aluminum! It's easier for you and your brothers to move, but even with rounded edges, you will hang up. Bad idea.

As for the snow machine. I remeber this one negative-degree night back in New York where my brother and I had the great idea to make some snow using the pressure washer we used to clean our dirtbikes. After getting the thing started, we aimed the nozzle up in the air, and blew a fine mist into the night. To make a long story short, it didn't work. I'm not sure of what backyard snow machines cost, but you can scratch the pressure washer idea.

That's all I have for advice off the top of my head. Just remember to measure everything carefully, and have fun building it and riding it. It's a rewarding experience building something like that, then having fun using it with your brothers and friends.

-Mike