an aluminum press plate - fifty cents each (used) at the newspaper printer
one eyescrew - for use as a handle, but don't use your hand, use tongs. a tongle?)
I made a template from posterboard, test fitting, and very carefully determining the size I would need to cut. Total length, plus 1cm for the folded seams, and the height. I "measured twice and cut once" (quote Mr Smith) - ("Thank You Mr Smith!" (quote Me!))
I very carefully (sharp!) bent the seams to hook together (see detail drawing) and hammered the seam down tight.
I used a nail to pop a small hole to start the eyescrew into, and screwed the eyescrew in about 2cm from the top.
I sprayed a coat of hi-heat black onto the exterior of the extender.
To use it you remove the cooking grate from the grill and put the extender into place. Fill extender with scrap lumber or twigs and light it on fire. Wait till coals are burned down, then remove the extender and replace the cooking grate. Ready to cook!