Breville/Sage Precision Filter Basket 16/20g - B62.52TH28E  £21.60

Fits my bottomless portafilter well. Very tight but lets be honest, that's betterer. Haven't tried it in the standard Sage one.
Less of a taper on the spout side than the Sage one so there's more flow, this has meant having to grind finer and dose the full 20g but I have noticed a small improvement in evenness of flow through the puck and less likelyhood of channeling. I recommend pairing it with the IMS shower screen, which made a much bigger difference but all in all, a good product.