It works. Narrative description left to DM.
tl;dr It just works, and it's up to the DM to describe how that looks.
The sculpt spell feature of the school of evocation subclass of wizards states what it does. It's only limitation is that it affects "other creatures that you can see". There are no secret rules and the feature does not state that it doesn't work when the creatures are grappled, attached, smooshed together, playing twister, snogging, etc.
Pockets don't need to be a certain shape.
Nothing about the definition of pocket [1,2] asserts the shape must be spherical, ovoid, cylindrical, etc. A pocket occupies space.
: a cavity containing a deposit
any pouchlike receptacle, compartment, hollow, or cavity.
Aside from having a volume, the shape and visual affect is up to the DM to describe if it's desirable or necessary to describe.
You might not need to describe it. If it's not questioned by the players, not important to the story, or not interesting and fun, skip the description and get on with it.
If it does need to be described, evoke the image of a mosquito.
The stirge is described as a monsterous mosquito. So when it attaches it's settled on it's victims skin standing proud supported by it's legs and stinger.
This horrid monster looks like a cross between a large bat and an oversized mosquito.
As the stirge is a monsterous mosquito, Describe the pocket of safety as wrapping closely to the character with the border cutting the stirge off at the stinger and legs. Failing the save and causing the stirge to die could involve a gruesome description of an oversized mosquito going splat with just the stub of it's stinger remaining protruding from the character's skin.