The choice is yours. If you want to use pins to hold things down, lay a ceiling tile (or something similar) on top, or you can use weights.

That looks like an excellent building surface, so you wouldn't need a hollow-core door (unless you wanted to make it bigger).

About the only thing that needs to be pinned to the table is the wing spar. If that were my building table, I would get a piece of plywood (the kind with at least one smooth, finished surface) and lay it on top (good surface up of course). You can even make it slightly larger if you like, but I wouldn't over hang the edges by more than about a foot.

Then, to hold down the spars, I like to use those clips that are made for holding electrical wires to studs in houses (See pic)

They will leave two holes in your plywood top, but... big deal. They hold a lot better than pins and you don't have to have a soft, mushy work surface that pins will push into.