Basically:
- no modifications to the gun/"buffer tube"/receiver
- sawed the front end of the A2 stock flat
- drilled, sawed and filed the insides of the A2 stock (between the "hole" and the storage compartment) open
- drilled&pinned it in place

The ballpoint pen shows the pin location.
The felt point pen shows where some ugly edge was unavoidable. The sharp edge can be rounded a bit but not much - the polymer surface is pretty thin.