I have used ready made since I started about 10 years ago. I have a -8 for one eye and a -7 for the other which are about 0.25 off my prescription. They are not perfect as I also use varifocal and have an astigmatism, but they do the job for open water swimming. I do not find it too much of a problem in the pool, although as someone does point out it can be tricky to read your watch if the numbers are small.
I get mine from Vision express who have been very helpful over the years. They, at least the last time I bought locally, also sell the lens individually.
I am sure you will appreciate the benefit of being able to see better in the water.