For some real trash-core DIY, I recommend using the free prio-mail cardboard boxes from the post office.
You don't even need to go in to get them, you can just order them free and they'll ship them free in bulk.
The medium sized ones make excellent shelves that can hold all manner of disc based format (and VHS if you're a dumbass like me).
They can also hold modestly sized books which is of course a plus for comrades.
Cut the closing flaps off the box and you can make supports so they don't totally bend in (I don't know if they'd actually cave in though without them, they might continue to hold but bent as hell).
Add a nice helping of duct tape to hold everything together and you're good, bone-apple-teeth.
One final word on the matter is that these become structurally shitty the more vertical you make them (shocker, huh).
What I've done is lean them back into boxes that are flat on the floor, and their weight holds them without sliding.
That probably only works for carpet, but whatever
They were actually fairly stable at four tall, but five started to present troubles, and six is obviously leaning.
Anyhow there you go, a shelf for the masses.