gwmeyer
Please comment on suggested alternate game pieces.
My team plans to build only one current and 3 gyres so the quantity for whole field doesn't always work out.  If someone has the Uline bottles, could you let us know of products at the grocery store that seem to match not only the general size but the thickness of the plastic.  If someone finds the same hula hoops at a store, please let me know.  I've already found some that are only an inch or two off and cost too much. 
I've already ordered 6 bait nets because it is only $10 more than ordering one when shipping is included.
Hollow Plastic golf balls are available at Walmart stores.
How about the mailing tube or boxes?  We will probably just make our own but if there is a lower quantity store option, it would same us some time.
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jgraber
In Dallas area, we are using 1 liter bottles for ~$0.58 each filled with flavored carbonated water from Walmart or Kroger.
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dannyk
We bought the hula hoops that looked similar on walmart.com. I hate walmart but it was the easiest and they had free shipping.
Using golf balls from last year. We're just using whatever bottles and cans we can find, that match the volumes. We bought the minimum order for the tubes and are sharing with other teams in our hub. Same with the boxes but you can probably find ones the right size somewhere else.
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jgraber
re hula hoop: Any flexible tubing will work for the hula hoop;  PEX or flexi black sprinkler tubing even should work.

re boxes: Boxes can be cut down from other sizes of cardboard.
The add-on tab with the hole may not be as sturdy as the game design team thinks.   They were thinking of jerking a freely moving box,  not lifting the tab while the box was caught underneath a shelf by poor driving.
If you are not going to put a box on the reef, exact box size is not important.   Just a cardboard loop to show you can group pieces together is fine.
If you intend to lift boxes, the right size can be cut down from other sized boxes.  

re bottles: Only the 1 liter bottle has much shape confusion.  There is quite a difference between a 1 Liter gatorade fat bottle, and a 1 liter soda thin bottle.

re gyres:  probably one gyre would even be enough, if you can move it to various spots.

re current beam:  I tested our game field with a 3/4" PVC pipe fitted like a pipe wrench around the beam.  Just 2 rolls of pennies on a 52" (?) lever arm twisted the beam by 10 degrees or more.   At hub game day, one adult built robot (built without a sample field) twisted the beam completely upside down due to being top-heavy, which was one reason the 8" max height above the beam rule was added.
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EvansPhysics
More about one of the alternatives for the hula hoops that jgraber mentioned...

1/2" Well pipe (aka. sprinkler tubing, polyflex, coil pipe, etc) is fairly inexpensive - under $16 for 100' at our local Lowes.  The OD on the brand we bought measures between .75" and .78".  100' should make about 5 gyres.

Link to Lowes
https://www.lowes.com/pd/ADS-1-2-in-x-100-ft-125-PSI-Plastic-Coil-Pipe/3502294

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