You just pencil out the dimensions and get cracking with a circular line saw.
You just replicate the dimensions and angle of cut from the old set of doors. It is pretty intuitive. But you have to put in this staggered cut in each - so as to keep rain from entering through the cracks between each piece. So, I cut in half the depth of the Birch at about 1/2 inch in distance into each board. So each piece slide together like puzzle pieces. Check the arrows, the complimentary cuts.
Trim the height to get it just right. So the hatch slides over properly. You can power sand the bottom piece to get it right. Or re-cut if the correction is large.
Then give it a light sand and she's ready for varnish.
I know there are a lot of varnish afficionados out there- I am not one of them. But I know this works for me. I lay 4 heavy coats of this stuff down and I am good for a few years.
And here she is all finished - the new doors give a cool, two-tone effect of Birch with Teak trim on the door.
Guidance and inspiration for this project came from Don Casey's classic sailboat maintenance bible, This Old Boat. It is certainly an uphill battle to keep an old fiberglass boat looking snappy, but this book provides you a blueprint for that battle.