There are a couple of ways to do this.
If you don't want to make any modifications to the trailer, you can lead lines up to the boat which will allow you to tie it down centered. This means that you need to avoid backing in too deep or the lines will go slack before the boat grounds out as you drive up the ramp. This is what is done on marine railways and works fine there but they are designed to take many different types of boats.
The next option would be a keel guide. It would probably be easiest to put a guide in between the keels rather than making a guide for one or both keels. Keep in mind that the guide can't be straight, it needs to start at a point and widen so that you don't have to be perfectly lined up as you drive on.
Another option is a gunwale guide. People often make these out of PVC and they work reasonably well. The advantage is that they are easily seen as you come into the trailer so it is easier to line up. The disadvantage is that they often allow more movement and don't work as well on many sailboats whose max beam is forward of the guide.