Just an update.
I experimented with a limited selection of material. Firstly with the diffuser material inside TFT monitors. I tried various distances from inside the TPU to just above the film holding pieces. Better, but the inconsistent illumination remained.
I had a scour on eBay but didn't find anything promising.
On amazon i found
this
I used the A4 sheet just placed on top.
I think i may move the diffuser down by removing some spacers.
No obvious illumination issues in first test scan. Not tested a colour scan but it's rare that I shoot colour on 120 film so not a priority.