first of all, the spruce, you can buy it in 1/8" x 1/4" strips from model airplane suppliers, then you can cut yourself a strip 1/8" x 1/32". it won't expand enough that you need to worry about it, and I usually soak it, and put it in the route and let it dry, it won't go all the way in, but once it's dry, it's shrunk back down,and will hold the shape (the truth is, you don't really need to even soak it, but it does curve nicer without kinks) then you can glue the preformed piece in.
As for the resin, it's a product that model railroaders use to make lakes and other "bodies of water" it's just an epoxy resin, but it doesn't have that yellowy tinge most epoxies have.
It hardens every bit as hard as plexiglass. The problem with using plexiglass and lexans, is not much sticks to them, the bonding becomes an issue.
As for the paper, since the resin is in liquid form, you have to lay the paper into the route first, I like to glue it down, then pour the resin on top, but do a test piece first, you want a paper that won't absorb the resin, or it will change the color.
Another little secret, to a plastic that will work, but is kind of a pain to use, is cut up a jewel case for a CD. that plastic is crystal clear, polishes up great, and is cheap