Get yerself a long ,wide ,flat piece of cardboard -such as a giant card board box- yer can find those in the back pf supermarkets or home depots or lowe's or sears !
Than get yerself a good razor knife- box cutter what ever yer will call it.
You will need a straight edge or a 4 foot level fer a straight edge-and yer angle finder fer yer figuring angles,- and make a pattern fer yer set up out of cardboard- call that a template. Fabricate a template fer each shelf.
Got it- card board- a good razor knife -straight edge fer yer cutting guide,I use a ajustable speed square but I am sure you have yer own method of catching them cornors !
Make a perfect template than build yer shelf using the demisions of yer template. Of course yer will require a roll of duct tape ! What yer will be doing is creating in the rough a full scale model of yer shelf.
OK so, five dollars fer a razor knife-should come with at least five blades, and some duct tape another five. The duct tape and the razor knife will come in handy on other home projects ,so consider it a investment.
Now if you search in the warehouse section of yer local appliance store-home depot -sears what have you- you must know that they get diliveries fer large appliances and other huge items-big screen tv's so on and on ,and those boxs are broken down then these days recycled-however-some trusty worker or even the workers manager will be glad to give you for free a few large boxs fer yer project templates !
Furnature stores , carpet stores ,dumpsters all that,
Fallow the creative trail- when materials can be free-go for it.
Now you have an idea.