thanks again everybody
magnum, in regards to filling tunnels, it's pretty self-explanatory. i do it in layers; the first layer has to be filled on top of something that the resin won't stick to, like wax paper or rubber. that way you have a solid layer that will prevent the rest of the resin from spilling out the bottom of the tunnel
QueerPenguins, working with resin can be difficult at times. it's messy, and you have to be precise with measuring the amounts to mix together, otherwise your pieces will come out tacky or soft. it's also difficult to mix without getting air bubbles into the resin. different resin types are more viscous than others, and if you get a particularly viscous type, the air bubbles might never escape once they get trapped under the resin. this is why industries use vacuum degassers--i don't have one yet but i'd like to get one (they are expensive, though). they remove the bubbles out by sucking out all the air to create a vacuum chamber.
there are tons of ways to go about making these. i'd suggest checking out the Smooth-On company website, since they make the products (that i think are) best fitting for this kind of work. the rubbers and plastics they make are AMAZING.