An approximate solution is to offsetsrf then wirecut with the perfectly shaped border the stuff that extends past it. The downside is the edges won’t be square.
Alternatively you need to do a multi-step process:
offsetsrf
determine how much extension there is around the perimeter
create a border that is variably inset that amount
trim original srf to new border
offsetsrf again should give exactly (or really really close) to desired overall dimensions