we use untreated pine shelves and cut them up to size. you'll probably want to have a platform in the middle as well so the chin can't fall to far. all my cages (apart from one that usually is a spare but my rescue girl's in it now) are very tall but they are divided by large pine platforms. the way we do it is have a platform that sits in the middle and has space on the sides. underneath that and above that, on each side, there's a pine shelf that overhangs the platform. so basically, if they fall off a shelf, they land on the platform. if they fall of the platform, they'll land on a shelf.
the platforms and long shelves are held in place by these little J shaped brackets - you hook the 'bottom of upside down J' on a bar and screw the flat side on the front of the shelf in several places so the shelf can't move.
we also have plenty of little ledges around the whole cage and they enjoy them very much.
all these need replacing every so often because the chins love to eat them but i quite like that because i know that if they've demolished their chews, they can have a go at their shelving.
i agree the bars at the bottom are too wide but i can't help with that. we did try putting mesh around the bottom of one cage when fluffy was due to give birth and was getting so stressed when i tried to put her into a cage with narrow spaced bars that i decided to leave her in the one with wider spaced bars and we were rubbish at it!
not sure about changing layout. i have found that they tend to move around quite often without looking and i've had a chin jumping onto a ledge i've just removed to replace it!!!