Well I have a few questions,
A: did you burn or scorch the plastics when you were melting them?
B: were you re-using other plastics to make them with? Or was it fresh "un cooked" plastic?
c: and if so, from what company did you get this from?
But to answer your question about scents, you can add it to the plastics just before you pour them. Garlic is a cover scent, not a attractant.
Otherwise we would use garlic cloves to catch fish.
Try crawfish, Or go for the PRO-CURE threadfin shad. But if your married, or have that someone special, use the garage or do it outside. It will smell pretty bad inside the house.
And about the hollow top part, I take it that you have one of Dels 2 part molds? If so, the plastic will "shrink a bit when cooling, and in doing so give you this tube affect.
All you do is watch the top of the mold, when you see it starting to be sucked down just add a bit more to the top.