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cash in hand beer money buddy at the shop deal.
a trick i used on my neck was I bought a 58mm cue ball well. 57.1mm like this
...

I took my upper to a friend to have welded up. ( ya welder friends)
then took it to the machine shop and had them cut the holes for 40$. took alot less than and it much more even square and perfect...

I looked into doing some of these.
https://www.ebay.com/sch/i.html?_odkw=phenolic&_osacat=0&_from=R40&_trksid=m570.l1313&_nkw=phenolic+sheet&_sacat=0
the material is inexpensive and can be...

thats not bad actually. I was looking at getting them made. and were going to cost me about 80-110 each.

maybee a smige more plenum ?

not sure exactly. some type of aluminum.
Dropped it off with a guy i have around the corner and he welded it up. no questions asked.
Then hit the holes with a drill press and flat bit .
Then used...

if you are going at this on your own. i have two things that really helped.

1. Ball hone on a stick. I got a 1.25 inch one that fit down the runners.
and a 2 inch one that helped me clean up...

Love the go no go ball idea. I will need to remember that.
I am going to post a few photos. of what i did. I am not saying its right. or wrong or correct for your application or not completely...

get the grinder out. or pay someone who knows what they are doing with one.

whats the goal? would be a big plenum be a better idea?