If this is not a stamp/seal, but an inscription, I suppose a normal quill pen will do. If we compare the width of the strokes where they are the most pronounced, they are not that much wider than some of the strokes in normal Voynichese text. Voynichese letters are quite tiny by themselves. Here's a comparison between You are not allowed to view links. Register or Login to view. top writing and the squiggle.
The blurry wide strokes of the squiggle require a separate explanation, but I think writing on a wet surface or a water spill shortly after writing could explain why parts of the inscription are wide and blurry.
The blurry wide strokes of the squiggle require a separate explanation, but I think writing on a wet surface or a water spill shortly after writing could explain why parts of the inscription are wide and blurry.