Not a huge fan of placing any speaker ‘inside’ a cupboard, but you’re the only one who can tell if it works for your ears. 

Where are you plugging in the power cord?

I’s suggest the Five over the 300, you can look at the product pages and see where the speaker drivers are in each and make your decision based on that.

We had a Play 5 sitting on top of our kitchen cupboards for years and were quite happy with it, in the cupboard with the doors open shouldn’t sound bad but I’d put it as far forward as possible.

On top isn’t an issue with me, I’ve had Sonos Ones on top of cabinets before. It’s inside a ‘closed’ space that I’m concerned about. Reflections of audio waves could be troublesome. 

‘Echo’

Sitting at the front edge, with the cabinet doors fully open it shouldn’t be much worse than a bookshelf placement. If it is pushed to the back of the cabinet it wouldn’t be as good.

If the doors are closed, even partially, it is going to impact the sound too.

As @buzz points out, a cupboard is a secondary speaker ‘box’, and may cause attenuation of certain frequencies, and potential echos. 

The OP is the only one who can determine if it is an issue for them, it is their ears that must be satisfied.

I’m still curious about getting power to the speaker. 

The cupboard has sockets so power was easy enough, I tried it with a Five, it was ok at the front of the shelf with doors open. It’s back on the stand now, I need the cupboard space for kitchen things… was trying to keep the mrs happy...

Do you have a spot where a Ray or Beam could sit out of sight, on top of the cabinets or even the fridge?

Wife is much happier with a Beam in our bath than she was with a pair of Play 1s.