assume you will need a square hole for the shower, I think your idea of an angle grinder is probably best. least likely to warp the tin, though it may heat warp anyway.
Either way I would tape it with some heavy tape and then draw your lines. The tape should prevent the tin from tearing as easily, though it will smoke a little with an angle grinder.
Had to cut quite a bit tin commercial building siding, and used an angle grinder for it. nice thing was we could smooth any edges with the grinder too.
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