jewellery making advice.

[mscate.livejournal.com]

I've just started making jewellery and really appreciate some advice on a couple of things regarding some trinkets I've bought lately.

1) I bought some little plastic boxes that i would like to make into brooches. the problem is that the backs are hollow. What is the best product to use to fill them that I can attach a brooch pin to?

2) I also bought

these

should I attach a collar to them and make them into key chains? necklace pendants? I love bunnies :-)