Train the brachialis hard.
Cross body hammers, hammers, reverse grip curls. The muscle lies in between the bicep and tricep.
Also focus on the lateral head of tricep. Rack press, CGBP, bench dips, JM presses.
I added significant “thickness or width” by doing all my back exercises supinated grip and then all my curls hammer or reverse grip.
Also my forearms got much better doing this. Try it for 4 weeks.