I have been known to stitch together many Frankenstein batts. I always use a wide zig zag stitch and if I am coming to an uneven area I will double stitch it to avoid a lump. I do my best to insure straight lines, on curves, in the pieces and it usually works out. I thinks we all end up with a lump or bump or raised area from time to time. Some times I find these are caused because of stretches in the batting. I try to cut those areas away. Best of luck.