you can use any fabric for an interfacing; muslin works well. and as for the wonder under or another fusable...you can use a fusable in your bag but it has to be ironed to something; interfacing is a fabric used to add a layer, giving body to your piece, fusable does not add that layer, it simply works like glue to hold pieces. 2 very different animals. but, any lightweight inexpensive fabric can be used instead of purchasing a separate interfacing.