Squishy plastic lava thing

remember these squishy things? You got them cheap at toy stores and were always suspicious of them breaking and spilling their noxious contents all over you and your family (at least, that is what I worried about as a kid.) I think I had a cd case with purple blobs of liquid. Anyway, I want to try making something like that, though I haven't done anything like it before. I suppose I need your help with ideas for
1. what would be a decent liquid filling that wouldn't degrade or get moldy or fade somehow

2. how I would go about sealing the edges in a aesthetically pleasing manner. I suppose there is heat sealing, and glues that bond plastics? And...I guess I would not be able to make very complex shapes?? 

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