Be creative. There are a lot of things you need to consider. I built mine out of 2x4's but skipped the ply wood and just laid a thick top sheet down with 1"x 1/4" steel on the sides. That was the least expensive way to go but I did have to put some support between the frame, running down the middle to reduce sag. YouTube - Backyard Box
Its only 15 feet long though. I'd make it 20 because it slides quick.
On that video from the previous post they used a design that uses a lot of wood and the thing will weigh a lot. If you go that route make sure you have a permanent spot to put it because carrying it around is going to be difficult. And don't use High Density Poly...Use UHMW poly. There is a huge difference in how it slides. High Density poly is also known as puckboard and it sucks.