the curve doesn't look that drastic, so i would wet mold the case from the bottom to 1/2 way between the middle of the curve and the top. the back would be flat, you can make the back long enough to make a strap coming across the top. but if you mold the case right(or even close to right), the case will hold the tool in all on its own without a strap over the top. personally, i like stuff curved like that more. you can make a molded case/sheath, and you don't need to make extra straps or loops to keep the tool/knife secure.
check out the sticky thread on the Horizontal knife sheath. same concept.
hope that helps.