My Highland mods
Oklahoma Joe firebasket sits on the firebox grate (grill grates removed to cook chamber. To me the most important mod for temp control and longevity of the Highland.
You can wrap the FB Cooking grates in foil and use as deflector/baffle I use the cast iron divider from my Primo Oval XL.
Exhaust Stack extension. Stainless tailpipe extension from Auto Zone.
No gaskets no clamps no second thermometer no portholes drilled
Firebox paint blistered and peeled. It eventually quits peeling. Before and after each cook for a good while I sprayed the firebox with cooking spray. Pam works. But store brands work. Treat it like cast iron. Don't spray while cooking or that spray will lite up. I spray FB when cold then again in warm down.
There are multiple YouTube videos on Highland. Fire control, full modifications, etc. They're good
I settled on stack extension, fire basket, heat deflector to push heat downwards entering the cook chamber and use cooking spray on exterior of firebox.