How to wrap the polythene around the crafts to protect them from dust without destroying their appeal?
Wrapping anything in polythene is inevitably going to have a profound effect on its appearance, there is no way around that. 

A better solution is to display them in a closed case or cabinet with glass (or similar)  sides or windows, these vary in design from very modern looking all perspex cases to more antique looking wooden framed ones (google 'display case' for countless examples. 

There are also other similar solutions, you can get glass or acrylic bell jars reasonably inexpensively. 

Two dimensional work can be framed behind glass.