gemelenMy guess that you need 1) a right pot - so leaves' material would have enough room and would be submerged 2) correct ratio of leaves to water for that pot
When dealing with pots for Japanese teas in a traditional way it's not only #kyusu but #testsubin too, which is likely more suitable and more available for 1L+ volumes.
As for cold water brewing it's easier though - any vessel would fit. And I recommend to try cold #SenCha