Sorry for the delay.
Mine has about a 2" lip left after cutting the hole.
It all depends on how big your cake pan is but I see no problem with using L brackets.
I use this metal cabinet the I found when cleaning up at my dad's house after he passed away.
Works great and reminds me of my folks when I cook.
Now I just need to find a Brownie to go along with it.
Maybe you could post a pic of your charcoal set up.
Mine will hit 350+ with the vents wide open.
How many lit coals are you starting with?
Is the airflow restricted somehow at you deflector?
All I can think of now.
Yes the second grate fits on the ledge and the pan ends up just below it.
Cooked a tri tip last sunday and it worked great.
Still need to build a charcoal basket but life is getting in the way.
I used a 10" x 2" cake pan.
$8 at Walmart.
I used the top of the pan for a template to cut my hole.
Next time I will use the bottom of the pan as it is slightly smaller.
That would make it easier to ensure the pan covers the hole and you have no direct heat path.
I've read countless posts on building these and here is what I came up with for mine.
Picked up a Smokey Joe Silver on clearance at Target, an Imusa pot at Walmart, and some hardware at Ace.
I've seen a few people use a pie pan for a diffuser instead of a ceramic flower pot base.
I went with...