Offset inwards yet stop edges from going inwards?

I have some aircraft cowlings that I must inset 2mm but I need the overall length height etc to remain the same.

As they curve in at the front as a compound curve that front edge area will get less. How does one offset inwards and stop edges from going inwards ?

My current plan is to clone the shape and extend surface then offset that but it causes problems on some shapes.

I dont want to have to explode my surfaces and extend them so need a setting to preserve overall size.


Tricky problem, it I understand it correctly.

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:
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