danncphedranyour going to need to know how to weld, or you can go to http://www.jibsforcribs.com/ or another site and buy them.
Not necessarily. You could build a pretty solid rail anchoring your steel coping in with bolts, and lay the topsheet on the wood. If done right it would be fucking solid. Obviously it might not last a decade but it'll last a while.
you could do the underlay with ply or just pick a board the width you want. Add 2x4s on the sides dropped down the height of your coping - the topsheet thickness. Bolt them on there good. Leave a little space on the outside of your top sheet for the material to expand.
If done right it can be solid. There are some of these in existence at mountains as we speak. A lot have gotten phased out or will eventually but they work fine.
No welding needed. You could build a 16 box for under 400 and that's paying retail for steal, wood and topsheet included. Honestly could probably be done for 300 or under depending what you have access to. A 21' 11 gauge tube costs around $90 I think. If you're alright with 4' sections you could get enough 1/4 hdpe for a 16' box shipped to your house for $80. The wood depends how overkill you're making this ans what width board's you're getting, if you're doing plywood skirting etc. But even if you skirt it on both sides shouldn't cost you more than $80(could be much less it depending on how you plan)
I wish I knew somebody close buy that wanted one. I'd be down to build one this style and do a tutorial with all the costs included to help people out.
If anyone in the northeast wants a box for cost or just over let's make it happen.