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Since I like a wider slot the some, i use one blade but two cuts. Andy.
"Slitting saw" is the cutter you're looking for- but... mounting it in arbor in a drill press, then ficturing the seat tube on the drill press table to cut a "good" slot will be a PIA and your result may not be what you're looking for.
Drill your 'pucker hole" them mark both sides of the cut with some tape. Use a hacksaw, and finish the slot to width with a file.
I, like Andy prefer a bit wider slot, but I cut the slot originally with two blades, and fine tune the width with a rotary tool. If a Dremel is all you have it will work but there are better tools available. Takes just a few minutes.