Talked with my friend that makes slings, and other gear. It was a full time job for awhile. He had not priced how much it would cost to make his slings today. He was buying in bulk for parts, and always looking for a deal.
Do you have the sewing machine that will handle your fabric? If not, I say buy it made already.
Have you priced the individual parts (webbing, attachment points, buckles, etc)? How does that compare to cost of buying outright?
The pride from making your own stuff may outweigh all of that.