It's not important. It's a little more convenient because it leaves less clean-up to do with a sander. If you follow the image link, you can see a few shots of one I fixed by using electrical tape to compress the glass and resin. After wrapping with electrical tape, I put a piece of poly plastic around the rudder and clamped two pieces of plywood on the large flat areas to get some compression there because the tape won't press very hard on the flat section.

The key is to insert a piece of plastic (or one of the scrap plastic bushings) in the hole in the rudder so it doesn't fill up with resin and you can locate the hole later. I used opaque white delrin so I could shine a light through the rudder and still see the hole...but the black plastic bushing should also be visible through the repair. This should help you reshape the missing piece.

I would sand back the remaining ends on the rudder to expose the surface of the rudder core so you know where your core roughly needs to be. Put in the bushing and shape up your new core with thickened resin and shape it to match the old core. Make sure the old glass to old core has some gradual taper to it and the start glassing...you can't have too much glass there. Be sure to spread the repair out at least 2 inches in all directions from the break. You will then need to make sure the thickness of the rudder will work in the casting - but there is a lot of room to build up the corner of the rudder without interfering with anything.