Belt Sander Seam Rubber

A Seam Rubber is used for smoothing Stitches and folding Creases into Canvas, it's like a Sailmakers Iron.
Get Some Wood

Find a piece of Hardwood long enough for your Seam Rubber and let it dry.
Split Into Quarters

Split the Wood into Quarters.
Use if possible an Axe with an wide Head like a Splitting Maul.
Rough Shaping

You could use a Hatchet to roughly shape the Wood, that way you won't need as much sanding.
Sand Rectangular

Sand the Sides flat into a rectangular Profile.
Sand a Wedge

Taper one End into a short Wedge.
Slim Down the Handle

Slim down the Handle to a comfortable Thickness.
Use a Roll of the Sander to create the Bevels.
Rotate the Handle holding at the Wedge to sand the Handle round.