scanning 5x7 colour negatives

I have an old 5x7 Cambo monorail view camera which I love using because of  its format, its lightness and mobility.     Unfortunately, I rarely  use it these last couple of years.  I did, however,  use it to expose  the last of the 5x7 Kodak Portra 160ASA sheet film that had sitting in the  fridge. This  happened was when I was in Swan Hill for the Mallee Routes exhibition at the Swan Hill Regional Art Gallery in 2018.   

This is the location photo of the early morning photo session at Pental Island. 

The reason for the infrequent use of the 5x7 monorail  is due to scanning problems, not the camera, and they arise because   I don't  have any  5x7 film holders to scan the colour negatives with. This is  due to the  Epson V700 flatbed scanner coming  with film holders  only up to 5x4. There are no  5x7 or 10x8 film holders.  Soup to now  I have been placing the colour negatives directly on the scanner's glass bottom and then scanning the negatives using the Epson area guide.   

The  scanning problems I encountered are  Newton rings and intense cyan images. The results are terrible,  and they are difficult, if not impossible to fix in Lightroom, for  many of the images. I have spent hours on the computer trying to produce a decent digital file. 

The obvious solution is a custom made film holder but I had no idea how to do it. A one off plastic one  from a commercial plastic company in Adelaide would cost an arm and leg.   Peter Lee suggested a film holder made of Tru Vu Museum glass with some matting, and he kindly offered to have one made  for me by his very helpful  framer in Orange.  

The glass film holder arrived last week,  and I  started trialling it after I'd finished scanning the 5x4 and medium format  colour images from the recent  Mallee Routes'  phototrips to Balranald.  This is a  scanned  negative from the early morning Pental Island photo session and imported  into Lightroom:    

There has been minimal post processing at this stage.  There are no Newton Rings and the cyan problem is not present. This is a big step forward. The above image does have  a green caste compared to the digital one above, and it is made worse by the Posthaven template. 
However   I can work on that caste in post processing.  

This indicates that the scanned 5x7 negatives will require more post processing work than the 5x4 ones. The Epson works well scanning  5x4 negatives. However, I can live with that  change in my workflow for the 5x7 colour negatives.