Drawing received from Farlan.

Ready to mark and cut sheet plastic.

Using the back side of the exacto knife, I drag the line next to the steel ruler.
I make 5 or 6 drags across the plastic.

After scribing the plastic, I can bend it back and forth and it will break on the line cleanly.

I scribe a line to the length I want the pieces to be.
I then use scissors to cut the pieces.
I could have made several passes with the exacto knife but the scissors are faster for short cuts.
 

Using my steel ruler and exacto knife, I make center X marks by setting the ruler across the center of the piece corner to corner.
Make two lines marking the center.
Then I use a sharp pointed tool (my 1/8" round file) to make a small punch hole.
Using this small hole to hold the pointed end of my calipers, I make 2 circles on each piece.
The inside diameter and the outside diameter f the frame.

Before I can turn the pieces, I cut close to the outside diameter line, making rough circles.

Time to get the bonding cement out.
I do not need to put glue in the centers as they will be cut out.
Using the inside diameter line, I put the glue all around the piece up to and past the inside line.
I put 2 disks together and move on to 2 more disks.
I temporarily clamp them to the bench to keep them flat and tight together.
Once I have 4 disks made up of 2 disk, I can glue 2 to 2 make a 4 layer disk. (1/4" thick)
Clamp it to the bench with a block.
Done the last 2 and clamp them to the bench.
The blocks happen to be my ballast blocks for the ALMA schooner and are made of 1/2" thick steel plate.
Will  let these cure over night.

 
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I will drill a small hole in the center using my center punch mark.

 
I will put the 2 disks on a long bolt and tighten up a couple of nuts so they do not slip.
I will chuck them in to my drill press and using my home made cutting tool, I will turn these pieces down to 3 1/8".

Starting the rough cut to make the disks round.

     
Cutting away, down tot he 3 1/8" line.
 
    
Once the 3 1/8" outside diameter is reached, I will use my exacto knife and a tool made from a file cut out the indise to the 2 1/2" line.
Being cafe to not cut all the way through the first disk (4 layers) all the way.

   
I haave cut the two disks.
I have sanded the inside diameter to the 2 1/2" mark.
I have sanded the knothces for the water and air passage.

  
I now have two completed circle frames 3 1/8" by 2 1/2".
Sides sanded and edges beveled.