Mitre Slots with tapers

I have a DW744 benchtop saw. Before the flames begin, I am quite happy with it and don't have the footprint for a contractor's saw. That being said...
The mitre slots on it seem to have tapers at the front and back that I can't seem to figure out. Crude ascii art:
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I cannot figure out any way for a mitre bar or runner to not have slop. If it fits in the front opening, as soon as it is past the opening there is slop. if it fits tightly in the center of the runner, it has too much resistance at the front or back opening.
Am I missing something, or should I follow my impulse and break out a file and strighten those tapers out? If so, what is the best way to go about this to reduce the chance of over-correcting?
Since this is a google groups posting, I'm sure I have been filtered out. If not, thanks in advance.
Jay
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wrote:

the top is aluminum, no?
I think I'd make a strip of wood that fit tight in the slot, a little shorter than the fullwidth section and a not quite 3/4" taller than the depth. seat it in the slot.
take a couple of pieces of 3/4" ply with nice straight edges and clamp them tight to the first piece.
use a router with a pattern trim bit to *carefully* straighten the slots.
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