Different idea for a batch rocket floor channel

Just had a thought while looking into why my batch rocket sometimes pulses. My tape measure must have shrunk when I marked out the primary air intake by at least 5 mm resulting in a tad under 15% primary air and 5 % p channel duh dumbass.

Now the idea I had I was thinking about making the floor channel integrated into the 45 degree angles on bottom of the Firebox.
Use two pieces of 50mm 316 stainless angle iron capped length ways with flat bar (making a long triangular duct) and mating with the door or holes in each side of the front of the Firebox for the air to enter the triangular ducting. At the port end welding on 50x25mm box section same length as the Firebox is wide joining up the two triangular ducts. From there having a vertical section to inject secondary air into the port.
Now a question or two. No need for exact science or experience required I'm going to try different things anyway.
What height and csa would be a good starting point.
And
Because the metal is on the bottom of the Firebox but cooled by the incoming air will the stainless steel hold up to the evil temperatures.
And
If In installing this type of secondary air supply may it cool the bottom corners of the Firebox enough to effect the fire.

By the way its a 4 incher heating a bath where the bath bottom is effectively the top of the second Bell with a water heating coil very near the top of the first Bell doing a great job of thermosyphoning the colder water from the bath drain back into the top of the bath. ( lots of water hammer to start with until she flows properly then no noise whatsoever and yes open end system no chance of squish boom effect)

Couple quick scetches to give you guys and girls the picture.