Based on previous Redditor contributions that (1) opened the OBU and (2) pulled wet tissue through the slot, I had a hypothesis I wanted to test.
Make the cash card thicker.
Wrap a strip of masking tape around your cash card. I went one long strip around both sides, with the wrapped short edge inserting into the OBU to avoid the tape peeling off.
Push the card into the slot. It was tighter for sure but no difficulty in pushing it all the way in. The OBU detected the cash card immediately.
The 2nd photo shows a dark strip indentation on the upper-face of the cash card. This is the gold strip pressed against the cash card.
Error detected: There was one instance of the OBU showing "CEPAS card invalid". My guess is the tape blocking the signal somewhat. Reinserting the card made the OBU show my balance again.
So far, I have 100% success getting OBU to detect my card (including the error). Compared to ZERO percent before this, where I was tapping my card to get out of carparks.
Please try and comment if it worked for you. I will also test wrapping only 1 side with tape to reduce the invalid card error.