I have used some in the past that had been resealed with a bag sealer...they did work fine for me. I didn't make them though was a friend that made them.
As Carlo said though, there are many different sizes available out there and usually they fit better than modified bags.
For example, my friend had made 250ml bags by shortening larger bags. Well a 250ml factory bag is narrower and fits better in most cases than a shorted wider 500 or 1000ml bag. I guess one could modify the width too though. Bottom line I would use factory made bags as much as possible and use custom if absolutely necessary. Also, a good bag sealer would be required...the material is pretty thick.