HI Rafael, the original kit tip tank was a oval shaped box of wood with rounded corners.. this days you can do the same faster and cheaper using pink or blue foam (extruded poliestirene) used in walls as insulation. After getting the correct shape you can fiberglass and paint it. Making them removable is a good idea. You can use the countless pictures and plans on the web for replicating them as accurately as you want them to be.

or...

http://www.fiberglassspecialtiesinc....ES/BUD%20NOSEN