Hope I'm not too late to the party. I am a naval architect (Webb '82) and did some structural work on GRP with stringers for US Navy minesweepers back in the late 80s.
As noted above, don't grind away a stringer. You will reduce strength AND create a stress point at the discontinuity.
Without running numbers and just shooting from the hip, I'd build up a round area 20% greater in diameter than the backing plate you intend to install. Make sure adhesion is very good between the new glass and the stringer. Fillet everything carefully to avoid corners and sharp edges.
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