BLIMEY , Not This Again .
Cuttem outta K&S toob, alumn or brass , 3/8 in .
Use a thing saw. the thin one ,
On the bench running a ruler along it, you can get four equidistant parrallel lines . then Markum 2 inch. Then Middle ( 2 x 1 in. ) on opposite sides .
The 3/8 in. In , on opposite other sides & ends. For angle Cut .
Get cut started on opposite sides at middle inch, aiming at 3/8 in marks . and so on.
Then dress angle cuts & Sq Cuts. Tack Glue sq. ends aginst board & edges. File Sets ( 6 ) even on the angle .Finally solder a 1 in spacing at aft Sq Cut .
If ya wanna do a Merlin one, then whack up a tapered 1/8 steel ' V ' to ram up rears & tap in flat . Likely AFTEr fittingum .
best I can do at short notice, used to ventilate injun bay , air OUT . or Zorst .