The dimensions are measured from the inside in inches.
(L x W x H): 21.0 X 11.0 X 11.5
That will work fine. Does it have a drain in it?
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Yes, but should I go bronze or stainless steel?
I believe it's a 48 qt cooler.
Also, I've read somewhere that I need a filter braid the same length as my cooler is this true, or would a 10" braid be fine?
You don't need a very long braid. Either bronze/brass or stainless steel works fine. Some people prefer stainless steel.