Honestly, as someone who is the queen of "I could make that!" I would suggest that you just buy a commercial wrap.
Firstly, the chances of you getting it right (i.e. supportive enough without being too heavy) are not 100%. I'm not even sure they're 50%. I don't say this to criticize, it's just that a wrap is a very specific thing, it has to be right or else you'll be in pain while you use it.
Secondly, for the expense and time you're going to go to, just buying a wrap will probably be cheaper.
Thirdly, you are only ever going to be able to make a stretchy wrap, and it's my opinion that a stretchy wrap is only really good for a smaller baby -- you'd need to buy a new wrap anyway if you still want to wear a baby who is 20lbs and up.
Just my two cents. And have i mentioned how in love with my Didymos I am? Yes, it's a stupidly expensive big piece of cloth, but it's still totally worth it.