First off, powderpak is hard to get in such a small amount like that and would be extremely expensive. Go with astroturf, and when you use it throw some dish soap on there and it slides incredibly. Make sure you buy turf with longest hairs (the grass) as possible, it will add more control. At home depot and lowes, you can buy them in 6x8 mats and just
Mix and match or you can buy then in certain dimensions, starting at 12x1 ( for example for the runway you could buy 12x4 and cut it if its too long).
Now im assuming your building this setup on grass, so the landing area doesnt have to be too big, unless your concerned about mud, then you should make it bigger and maybe put some down under the rail in case you fall off the side of the rail/box. One important thing to do to make it work the best is, since its on grass, make a runway out of plywood on the grass thats just a flat surface that you can attach the turf to. On the grass the turf tends to dip and you get caught amd wont go anywhere, no matter how much dishsoap amd water you put on.
For the tower, 10 feet is too tall, 8 is perfect of you get the angle of the drop in right. Make the top 4x4 so its stable and theres
Plenty of room to move. For the box, make it as long as you possibly can eoth the space you habe. 10-12 is ideal. 8 is a little short but would work good with a double barrel rail witch you can make out of pvc pipe. Last thing i can think of which is usually the biggest question people have is what to put on top of the box that slides. The parks use HDPE, but it can be expensive to buy and get shipped. You can buy trex decking at home depot
If they have the right kind as well. But the best thing that i think there is to use for a home setup is this pvc decking stuff sold at lowes. Its cheap and they usually sell if in different dimensions
Hope that helped