What is the most cost-efficient design of a 16' temporary wooden ramp?
I'd suggest doing something like this:



Since this is a long ramp and your plywood is going to be joined in the middle, I'd suggest cutting your 2 of your 2x8' in half, then use full 8' lengths to bridge the plywood gap and using the 4' sections to extend them to each end. You'll have to brace each juncture (2x4) a bit more, but that can be done with a little scrap from your other cuts. This ramp has the double cross brace in the middle, which is where your plywood should meet. The only end of the plywood that shouldn't be fully supported is the one that meets the ground.

Don't forget the cross bracing between the 2 sides as well as the diagonal bracing on each side. The pic doesn't have diagonal bracing on the sides, but the taller end will need something to keep it from parallelograming out of shape. Besides the diagonal brace at the back of the ramp, like the pic shows, there should be at least one other diagonal brace on a ramp this long and tall going in the opposite diagonal.

The pic looks to be 2'-3' tall, based on the "human for scale" who is not really very close for scale, so this should scale to 16' long and 4'-5' fairly easily. It even looks like a decent grade, so you aren't pushing carts straight up.

Also, don't forget to put wheel guides on the sides of the ramp. You don't want a cart or dolly to fall off it at 4' high. That's a disaster waiting to happen, even without OSHA involved.

You might also want to at least consider adding some "outriggers" at the tall end, so it doesn't fall over if things get a little rough. That can be as simple as some more diagonals from the top of the ramp, perpendicular to the ramp itself, with some more 2x4 at the base to tie it in.

Even though you plan on having those ramp ends attached, you don't want to skimp too much on the safety aspects. If you're filling that whole container, you're going to be walking up and down the ramp very many times, probably with heavy things in tow, and even screws have a nasty tendency to rip out. You don't want to "pause" your packing for an emergency room trip.

FYI, OSHA regulations are mostly basic safety precautions to protect workers from complacent and cheap employers. There's not really much from OSHA that's overly cautious.

One last thing: your plywood should be at least 1/2" thick, nominally. Less than that at even 3/8" thick is not likely to hold up and 1/4" will break through almost immediately for anything of any significant weight. I was going to suggest 3/4", but that's overkill. You can also go with B grade ply, since you don't need the sanded smooth surface of A grade, and the knots of C grade might not stand up to the abuse of walking on it that much.

I know this sounds like overkill, but even a metal ramp that's unsupported in the middle can get serious wobble and bounce to it that will absolutely destroy a wooden ramp, unless it's made out of at least 4x4s or 2x6s. Even then, without 16' lumber going the full span, you're risking a lot on this ramp.