Focus Cloth Dimensions

Well, I finally did it. I left my focus cloth in the mountains somewhere. It was my homemade one fabricated with curtain blackout material and black cloth. Ii was a bit clumsy (rectangular 3 ft x 4 ft).

Has anyone got a guide as to the best size for a focus cloth. I need to get working on one but I thougth I would ask for suggestions.

I just have a manufactured one, about 2' square. I'd grab a black bath towel and go outside, see if there's a perfect size for how you work? 4' seems a little much unless you're wearing like a babushka??

Ultrabounce is a good material which is opaque enough for camera work, and water resistant. black on one side, white on the other. looks like ripstop.
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start with one yard, everyone works differently. for 4x5, I use 2 t-shirts sewn together unless it's really hot out, then I go with the ultrabounce.