I'm not sure if the materials it is made from are suitable, and if not is there a way to cover/protect the inside so the degus don't manage to chew anywhere? There's lots of pre-drilled holes on the side panels for fitting shelves that they could potentially get their teeth into, and also small ledges around the glass panels they might chew.
It looks like a beautiful bookcase!! I don't like it for degus,though. It is too shallow and too narrow, and although the height is significant, it isn't overly helpful to your boys. The materials . . . seeing them listed above . . . give me pause. There is an awful lot of plastic and foil in that, and with the pre-drilled holes that allow for the personal changes in height according to each owner, you are quite right that your boys will go after them and chew.
I am assuming that the idea of using the bookcase is because 1. the use of it would save you money. 2. You have it, so why not use it. But . . . if it isn't a good match (I don't think it is, really), do you have other things in mind?
I'd like to ask you some questions, to understand what you are looking for and what you are working with. What kind of space do you have available for a DIY cage . . . or any cage for that matter? What about finances, do you have a budget or are there limits you must keep to? What do you want out of the cage . . . easier to clean, to move, more running space for your boys, keeping substrate inside better so your house is easier to keep clean around them? Do you want doors on it? Are you familiar with the cage size recommendations that we have for degus on our forum? ( deguworld.proboards.com/thread/9396/forums-cage-size-recommendations )
There is so much that a cage can be . . . so it is helpful to know and understand what you hope for, so that we can better advise you.
It is lovely to meet you, Emmakeyes!! Welcome to the forum! I hope we get lots of opportunities to get to know you.
1 Bichon/Shi-t-zu Ollie (b'gollie), 1 Standard poodle, Indie