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A jig underneath the belt could work. Interesting.
Otoh, if you freehand, just don't use too much pressure, use a sharp belt and let the belt do the work.
As long as you don't linger in one spot,...

No you don't.
Atm I stil use the 1.5 inch wheel of my belt sander for grinding hollows.
You just need to 'roll' the blade from spine to edge back and forth in order to take metal out of the middle....

Are those 2 statements related?
Because when it comes to being useful, a true wedge is not that great in the 'keeping it sharp' department unless you work with tape. And even then... A minor nick or...