I plan to make it out of stainless steel, I would love to make a disk for you, but I'm afraid there will be difficulties with delivery, since I live in Russia.
Can't you simply send a letter or a parcel or is this meanwhile forbidden??!!
don't forget to take the mounted tyres into account. only using the rim diameter is only a rough assumption.
Compared to the wide tolerance range from 17" to 19", the tire dimensions should be neglectable as the width / height ratio is anyhow the same! 120/70-19" and 120/70-17" - Yet you are right the relation changes a little bit:
Wheel diameter 19" with tire: 19 x 25,3mm + 2 x 120mmX0,7 = 650
wheel diameter 17" with tire: 17 x 25,4mm + 2 x 120mm x 0,7 = 600
Calculated number of holes for 17": 48 / 650 x 600 = 44,3 = 44
So Franko is right, it makes one hole difference, that why I wanted to know the number of holes at Tills 21" version, but the one hole should not be a road block!