Your two options are either a sump baffle plate (cheaper) or a proper baffled sump.
The plate would probably be noticeable as I expect the edges will be visible between the block and the standard sump. But I'd still be surprised if an insurance assessor would notice something like this.
https://www.ebay.co.uk/p/Subaru-Impr...j25/1523877603
The better but more expensive option is a proper baffled sump - they look visually identical to the Subaru OEM sump if you get the right one, it's just got the right gubbins inside
http://lateralperformance.co.uk/prod..._single_scroll
Expect to pay £400-500 though before it's installed!