Tomas Slancik

ajz3d
First I created a udim_tilefor each SOP primitive

you should be able to just use MtlX UDIM Patch node directly in the shader to get UDIM number from provided texture coordinate value

malexander
(but unfortunately I don't know the syntax of that token)

in File COP you can use
however if you are generally trying to insert the currently cooked UDIM number within some expression then use

bbox() function gives you axis aligned bbox of the geo, it doesn't matter that the geo is the result of fitting oriented bounding box to other geo

you can however output Radii Attribute from your Bounds SOP to get the XYZ half size values for the fitted oriented bbox