OK, so what tools do you have already that can take off a slither? i.e., not a jigsaw.
A mitre saw would be what I'd go for. You can clamp a scrap of wood to the fence, right up to the blade, then you put the workpiece up to that clamped piece and the amount removed will be the kerf (thickness) of the blade.
Even smaller amounts can be taken off by pressing the workpiece up against the stationary blade, then raise the blade and come back down with it on, you'll remove about 0.5mm each time with that technique. It depends on the play in the saw's bearings, etc.
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