Help with homemade pannier bags

Im looking at getting some pannier bags for mu comute but all the ones sold in shops are expensive. So im thinking what if I do a homemade sack. I just need something to carry all my school stuff, and classes start in 1.5 months.

Im thinking I could get a big bag, like a gym bag, add some aluminum or steel plate to make it stiff on alls and bottom, then bolting it to some hocks to hang on my rack.

Any ideas, any one else done this before? any thoughts on the bag?

you have the right idea,just get a couple backpacks,one for each side and add some metal strips or something to stiffen the bags up and mount them.
im using a small black plastic basket on each side and they are mounted using metal strapping on my rear rack.