A Seam Rubber is used for smoothing Stitches and folding Creases into Canvas, it's like a Sailmakers Iron.
Step 1: Get Some Wood
Find a piece of Hardwood long enough for your Seam Rubber and let it dry.
Step 2: Split Into Quarters
Split the Wood into Quarters.
Use if possible an Axe with an wide Head like a Splitting Maul.
Step 3: Rough Shaping
You could use a Hatchet to roughly shape the Wood, that way you won't need as much sanding.
Step 4: Sand Rectangular
Sand the Sides flat into a rectangular Profile.
Step 5: Sand a Wedge
Taper one End into a short Wedge.
Step 6: 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.