Saw this in an email from Martha Stewart and had to share since it's a fantastic craft project.
Tools and Materials

• Knife
• Loofah
• Waxed paper
• Tart tin
• Ready-melt soap (such as honey-glycerin and honey-oil mixtures)
• Double boiler (or microwave)

Loofah Soap How-To

1. Cut a half slice of loofah using a bread knife.
2. Place slice in tart tin with cups lined with waxed paper.
3. Place brick of ready-melt soap in the top of a double boiler. You can also melt the soap in a microwave.
4. Pour enough soap into tart tin until loofah slice is just covered.
5. Wait about 30 minutes for soap to completely harden before popping it out from tin.

Resources
Whoopup Creek Luffas
96 RD 255
Glendive, MT 59330
Phone: 406-687-3874
Fax: 406-687-3074
whoopupcreekluffas.com

I would add some essential oils into the melted wax before pouring or even soap dye.