since you asked specifically about back carries, i would say a mei tai or a SSC would be the way to go, definitely. i'm pretty much an exclusive back carry mama except for when they're wee tiny (and really, i do it even then, just not exclusively), and use mostly mei tai's with the under 25 pound set and SSC's with babies once they get heavier than that.
there are loads of links to DIY instructions on the DIY forum at thebabywearer.com and there's also a yahoo group for folks who make baby carriers and there are some files there that i found helpful. there's a good SSC one IIRC.
if you sew at all, a mei tai really isn't that hard. i've made several. i tested the waters making a few ring slings and pouches, but since i prefer mei tais and SSC's, that's mostly what i make for myself now when i want to make something new. any easy first mei tai would be one like the BBO that has wide unpadded straps. there's loads of folks who've been making them over at thebabywearer, lots of threads with tips and pictures for inspiration. i personally prefer padding in the straps, but i tried a BBO with an almost 30 pounder last year and it was quite comfy. once you have that one under your belt you can always do another and try it with padding if you want to