Maybe keep the mg in the variable name? Also why a unique pointer instead of just using the destructor to clean up?

Adding a "mg" feels redundant. Propably something like httpServerContext.

https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rr-ptr

Here the pointer is private and no copy can be obtained, therefore the Interface already serves as "unique pointer" or in other words is unmistakably the owner. Basically this feels like double wrapping.

Externaly yes. But inside of RL::Interface member functions thous things are possible.

Stoping the context is the responsibility of the object. (If I would redesign mongoose in C++ you wouldn't have to manually stop the context.)

I'd like to use the destructor only for things where you need multiple member variables.

https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rc-dtor-ptr
https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rc-dtor-ptr2

Well, guarding against internal misuses seems somewhat futile an undertaking in the first place, at least I find increasing size to achieve it questionable. Guess would be nice if one or two more people could voice there take.

This patch is there to solve an internal misuse.
"Size increase" of what?

Source size obviously but that can be ignored for all intent and purpose. Binary size should increase while the worst case of size on the stack/heap with how you crafted it (empty deleter) should have been avoided.

Guess the binary size increase could be more than you expect.

Since std::default_delete is an empty class implementations should optimize std::unique_ptr for empty deleter.
Binary size doesn't matter in parts where it's seldomly called (loaded in to cache).
To reduce source size i could use a function reference as deleter but that would increase stack usage.