Quoted from GCS2000:
I picked up the Brian Allen Williams Mash up flexible translite - I assume I need the 90's Williams/Bally frame for this?
You need a frame the size of what you bought. Go to the manufacturer and find out the product dimensions.
https://www.flylanddesigns.com/product/williams-pinball-mashup-translite/ says 14”x20”. Is that what you bought?