It's certainly fine to use with the Sunshine Kids' Radian! Actually, it's shape is so seat-specific that it wouldn't work well in any other seat anyway.
Although you should never use anything that didn't come with the seat, waterproof pads are generally so thin you could certainly use them without in any way altering the tightness of the harness on the child. There are several aftermarket products that are quite trim and would work, but honestly, if you have one handy, you can also lay an unstuffed pocket diaper flat in the seat and achieve the same effect.