OK so the iPad 6 and adaptor have arrived and I have enlarged the slot so that the A1468 adaptor now fits. I don't have a dremel hence used a nail file (for the flat bits) and carefully used a larger drill for the rounded ends of the slot - these worked fine.
So here is the A3 with iPad 6 inserted (no velcro and no painting at this stage):
Knowing about the instability of the A3, one of my wife's first questions was 'can I remote control it so I don't have to touch it?' A quick bit of research tells me that you can use 'accessibility options' on an iPhone to control an iPad (on same network, with same Apple ID) but it is very limited - the only useful control was volume. (This may be more useful for music playback than TV, with next track, play, pause etc).
However, here's the problem: The second-hand iPad that I purchsed on eBay arrived on Saturday and also came with a children's iPad holder in a rather fetching colour; I had intended to throw this away as not needed/wanted. Having received and set up set the iPad up on Saturday, I temporarily fitted it into the children's iPad holder whilst waiting for the Apple A1468 adaptor to arrive. Here's a picture:
Bearing in mind that I wanted to use the 'new' iPad in the A3 as an extra TV in the kitchen that could be moved around (table to worksurface etc) and generally 'abused', the pink children's iPad holder is actually far more suitable! It's very stable on any surface, can be hung from a hook, is easy to handle, easy to charge (lightning), and easy to operate. (It's easy to find also because the garish colouring seems to stand out!) The only downside (apart from the looks!) is the sound quality.
I'm not sure now how to move forward. I might get an iPad 1 and stick VLC on it as Matador suggested- it will be far more acceptable to the discerning viewer/audiophile. I could use it in the garage ...
