yeah i was going to say that sounds way too short... i had one that was 8 feet long and it sucked. i gave it away.
this is the cheapest way to do it
find something that looks like it would be a good box, i went to the gas station and found two 4 foot plastic display boxes (they used them to display windshield wiper fluid, oil, etc in front of the pumps/store) they had some they were scrapping. they were good on their own i used to do lines hitting both boxes but it got boring fast.
so once you found a "base" all you need is 1 sheet of plywood which you can cheap out on if you arent doing slide tricks on it you can get it for probably 15 bucks. since u said u want it to slide you are gonna have to spend way more money like 50 bucks for the sheet. dont use nails i find them to be a pain inthe ass. get a 3/16 drillbit and some #10x1 countersunk sheet metal screws that will be much easier.
dont use circle coping, do you even know how to install that? you have to install it from the bottom man are you ready for that? fucking pain in the nuts. it means you gotta build it from scratch. 2x4s are the most expensive and are hard to cut nicely unless you have good tools which are mad expensive.
dont get aluminum, galvanized, or stainless steel they are all too soft and galvanized steel will eat your trucks alive im sure. its all rugged and shit. just get the plain steel if it starts to rust out (which it will) you can just do a few grinds to wear it off or if ur a baby get a wire wheel and a angle grinder and its good as new underneath the rust, you can do this a ton of times.
so my method u only have to spend money on a drill if u dont have one, (you should get one anyway they are useful), the screws (maybe 5-10$ depending where u go. you should look into getting stainless steel screws they dont rust) angle iron (should only be like 20 bucks tops for a 12 footer), and the plywood to cover the top and mount the angle iron. i ended up buying 2x4s just to hold the 2 boxes together better we driled em along lengthwise underneath.
so yeah steal a foundation, steal ply wood if you can, get a drill cause you need one for life or borrow one(makitas are best but dewalts are cheap if you are never gonna use it), get 10x1 s/s countersunk sheet metal screws (also called "flat head", fastenal part 1173032), and get angle iron (and get them to cut it most places have a bandsaw or chop saw so measure first) and youve got yourself a box for under 50$ easy.