Might be difficult to change without removing the cross shaft, but I agree with pinching and slight folding to work it in. I would warm a cup of water in the microwave and put the nylon washers in it to warm them up before installing them.
If you have the housing drained and top removed where you can get to the cross shaft then I would remove it then install the orings and washers. You should be able to disconnect the shaft internally and slide it either direction to accomplish this.