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Griffin, we were using a 3/8th's inch bolt (spaced with washers) to attach the tripod head. If you wanted to attach a camera directly, you'd want a 1/4-20 inch bolt (also spaced with washers).
And you're right, the skateboard wheels and bearings DID cost us $40, so this slider would be WAY cheaper if you already had that stuff.
It is amazing :)
Though to be fair, and honest, it has nothing on our sliders in terms of quality, functionality, or how long it will last. But we wanted to create and share something which worked when you had no other option, or were on a really limited budget. We don't really expect to see professional videographers using this DIY option unless their in a real bind or want a 'disposable' slider for one reason or another (think using it in saltwater, for example...)
We'll discuss it and maybe create a tutorial down the road for something like that. The big danger with less stability (especially with this DIY system) is that a small slant means your carriage and camera will move quickly and run the risk of falling off the slider.