My MRP always rubs in either extremes of gearing but with it being plastic you don't notice it.
I now actually run a 9sp cassette but with the smallest cog removed, and a spacer behind the biggest cog to take up the slack. I get a better chainline and with a larger chainring I find the gear spread perfect.
Hope you get it sorted. I wouldn't bother with B&Q though, you need a washer that is big in size to help keep the cage stable, but needs to be M4 so any off the peg are likely to be too small and made of metal so difficult to trim. I lost my spacer but found a lid off a ice-cream lid is much better as you can shape it to fit around the sliding tab.
Is that a lynksey built frame? If so you won't be the only rider at Bontrager who usually rides a SS Lynskey but has swapped the dropouts to run 1×9 specially for the event using a MRP 1×9 chain-guide! Shimano cranks all the way for me though, simples 😉