Vac 50 will work fine for what you are wanting to do. I have used it and have made a few injection molds out of it.
I have used a few things for making the boxes to pour into... corrugated plastic (cardboard made of plastic), lexan cut to make the size boxes I need and also legos. The kids are all grown and I had a bunch laying around so with those you can make any size box you need and dont have all the materials for the boxes laying around after your done. Just clean them up a little and put them back into a plastic container. You will want to use some type of release agent on your boxes and the item you are replicating. If not both will be a loss.
1. Prepare box to the size you need, hot glue the box to a smooth hard surface (lexan is good)- make sure you have depth for both halves of the mold that you will create. The first one I did I didnt make the two halves of the mold thick enough, dont skimp on material.
2. Spray release into box, and brush onto bait
3. mix your vac 50 (comes in two container) and pour into box
4. brush vac 50 onto the back of what you are making so that you dont get air bubbles in the mold under the bait
5. settle the bait into the vac 50 so that you have half of the bait in the vac 50, let cure just a little bit and push something like a dowel into different spots in the vac 50 for making sure the two halves will go together properly
when the mold is done and let sit over night.
6. Clean up top half of bait and whatever you need in the vac 50
7. Spray the top of the first half and bait with release and pour vac 50 on top. Let sit till cured.
You may have a little hassle getting the two halves apart but once you do your good. Just take you time at it, they will come apart.