You get a great bin cage from IKEA biggest Samla bin size they have. This bin is high and I doubt that your Robo would ever be anywhere near the top lid where you would cut a rectangular shape and place mesh on the underside of the lid. The gaps where the cable ties go are tiny, just big enough for the cable tie to fit through. A Robo or even minute mouse would never get through. Your mesh would have to be a small size.
This is an example of a video you could use but there are tons on YouTube.
https://youtu.be/i5e-2yKLX2Q
You don’t need to make any ventilation on the sides at all if your ventilation at the top is big enough. You need enough ventilation so use the lid to create that ventilation and use mesh.
I have included a picture of a small travel box for my hamster but it’s the same as what you would do for the bin cage.
I am not too sure what you mean by putting a smaller cage into the bin. It might take up unnecessary space? Why not use the space in the bin to create an amazing habitat for your hamster and leave the small cage for a travel cage?