In developing the SMPH model for predicting IBUs, Holsom found that the use of a hop bag did not have an significant impact on IBUs. But that's for a mesh bag, not a basket.
I read his writeup. Unless I missed it, he never describes the bag except to say it's a "nylon mesh" bag, i.e. there's nothing about the dimensions. I strongly suspect that the bag was large and fairly closely conformed to the kettle dimensions, effectively not constraining the hops.
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