Inventory Slot Material - math nodes

Blur? As in - they stretch? If all you want is a frame like above, perhaps it would be easier to use 9-Slice? Scroll down to Setting Image States:

ranging from 64x64 to 384x192

You’d need a single 64x64 png to cover all sizes.

Considering its spawned hundreds of times, being the slot of any inventory.

Consider the Tile View widget.