Hi Michael,
I think what you will find is there are different whites from maker to maker so colour matching the ABS may be difficult.
In saying that, perhaps get in contact with some of the vendors here and ask if they have any cut offs you could buy.
The link below will give you a list of the vetted makers to contact, but also reach out to Mike from Trooperbay as he also makes kits an may be able to help.
Another option would be to google local plastics manufacturers, some may offer smaller sheets of ABS if your lucky.