I may know someone(disclaimer)who made a 15m slider for free. And a 25m for free too. Not too hard to find the materials but it was all possible in a french plumbers van. The most succesful one was 8 borrowed pallets and 4 borrowed highway rails (The M shape ones) And 1 set of old fly lines. 1 box of candles and plywood boards for durability. I broke 3 boards before working out that one.
Knock the top 2 boards of the pallet, Then sit 2 pallets in a A shape. Tie off the bottoms so they dont spread. Lay the M rail on top and tie down onto the X at the top of the pallets. Becomes super stable if you can tie good truckie knots. Wax the top regularly as its way more slippery and will protect it from rusting. Rust will eat through any board base.
It will be semipermanent and needs to sit in 10 - 30 cm of water.
You will need 10 people to move it around.
Use 5 line kites cause I saw 4 suicide leashed up people badly injure themselves being dragged into the slider and 2 times looping kites trash it.
If any one asks,,,,,(disclaimer)its your own idea and you didnt get any ideas from me.