Dimentions of the opening of Steher nucleus "Simplex": 15 x 1 cm total
Opening for bee's to pass: 4,2- 4,4 mm (blocks also Vespa Velutina/yellow legged hornet to enter)
Reduces the opening with 1 or 2 thirds iff wanted - prevents robbing
Can be uploaded in a CAD editor and easily made wider/more narrow for other standards than Simplex
Tested - worked fine (I printed it with no supports)