I went out to the garage and checked.  I bought the flag here in town but here is the company that puts them out.  Click here: 
Firestik flagMine is the 4 foot model and came with a mounting base which I secured to the yak with stainless steel hardware.  The MSRP is 15.99 on the website.  
Also, I rigged up a PVC pipe and secured it to the milk crate as a mount for the aft navigation light.  The light is a  
Tecktite MK III and is an adaptation of the  
Kayalite.  The light fits inside the PVC after slight sanding of the light's plastic body.  I topped the PVC with 3M Solas reflective tape and refelective orange tape from Ace Hardware to give me a bit of safety comfort after the sun goes down.
I purchased my milk crates from the Duluth Trading Company so that I would have some that are not stamped with "PROPERTY OF...DAIRY".