Is there any problem cutting mylar bags down and manually sealing the edges? Assuming you seal it well enough will it still be as strong as the factory seal? I know it's a stupid question but when it comes to food my family and myself might have to rely on one day I would rather make sure than be in a situation with bad food and it's too late to do anything about it.
I wouldn't see a problem with it, but I would search and find a bag the size that was the proper size. Use a quality sealer and it should be good to go.
I have a bunch of 6 gallon bags I would like to use instead of buying more. Next time I buy bags I'll get a few different sizes.