I made a mount out of old credit cards.I cut a slit into two of them,&cm up & 10cm deep.
I then glued two more end to end so there was a flush fit.
Next day I glued the cut cards to the double length one.Each was 5cm from the join.
When dry,Sat was placed against upright two,I centred it & marked it at the side & front.
I then glued two supporting stripsof wood 5mm high,5mm wide &18 cm long for side supports and 2 small pieces 2cm high & 5mm square.
Next day when all the glue was set,I tried it in the card holders and added sat.Perfect fit.I have since spray painted holder black.
I know this sounds a faff,but it is worth it as the holder is very light and can be put in glove box or left in cup holder (noone will guess what it`s for) & take sat with me.