gap filling construction material

[svouts]

I need to constract a wedge between a vertical wooden post and a leaning garden brickwall. The gap is 1-3cm wide and 50cm long. The post adds support to the wall.
Shall i use mortar, grap adhesive, any sot of resin to fill the gap? Any other materials?

 

[r896neo]

pouring in a slurry mortar might be best. Obviously you'd need to shutter it.

A semi dry mix packed in would also do but perhaps not as throughly