It is possible to get even development with plastic tanks. I had the same problem with uneven development at the edges of the film you describe. It was even worse with 120 film. It was clear I wasn't getting the same agitation at the center of the film. I went to sliding the tank back and forth on the table top for 10 seconds each minute. I slid it as hard as possible, just to the point where developer is almost shoshing out the pour holes. This worked, but I went to stainless tanks and inversion agitation anyway, and no more problems. Enjoy perfecting your own method.

Best regards,