Its rather simple to cut a fin out of wood and taper the leading edge and trailing edge and call it a fin, and it will even work well to keep the tail of the surfboard from sliding out.
However designing and fabricating and foiling a wood, or fiberglass, fin with a good clean foil on both sides without flat spots, and a wood one which is not going to snap off right at the edge of the fin box is considerably harder. Not to mention a well foiled fin with a good template for the particular board will make its performance skyrocket, compared to the merely tapered edges of the theoretical fin in the first paragraph.
The grain of the wood, if not plywood will want to go from the base to the tip and not fore and aft. Even if plywood, well it too usually can flex more in one orientation as opposed to the other, and I want the fin flex to rotate the tip, easier than tip over laterally.
There is a multitude of ways to get enough strength from the base of the fin through the fin box up into the fin. it is very easy to underestimate the lateral force which can be applied.
My first wood Longboard fins made in 2002, which snapped, I still have most of them as I used Stainless wood screws from base into fin, which simply bent when the wood broke. The stainless however is not stainless when encased within epoxy. The one Sapele White Ash and Wenge fin that I made with enough fiberglass up the sides that it did not break laterally( the screw tab did) the stainless steelk screws rusted out and rotted the wood, I replaced them with carbon rods when I rebuilt the fin moving the screw tab to the back of the fin.
That particular wood fin performed very well, but pales in comparison in every aspect to what I now prefer to ride in my favorite longboard. Which is MrMik’s 3d printed gullwhale 7s fin, and the strength of these from base into fin and the strength of the retaining tabs are now mostly sorted out.
It is extremely difficult to actually hand shape, a precise foil or anything resembling one. My contour gauge on my previous fins have me hang my head in shame. I used the shadow method foiling those and they looked awesome. The coutour gauge reveals flat spots and bumps. But the fins still worked pretty good, so there’s that.
If you value your time, then making a wood fin is time better spent making the money to buy a fin from someone who has the process down. And it is a serious process with many many steps to tick every box regarding strength accuracy and how tightly it fits into the fin box. I am not offering to make one. I’d want hundreds more than anyone in their right mind would spend on a surfboard fin and would likely still make more at a minimum wage job.
It is much better( in my opinion) to have the whole fin base in full contact with the box, rather than at just the roll pin, screwplate and as few as 4 other points of the fin base. The whole surface area of the fin base, touching all of the box interior adjacent to the fin is pretty much impossible to achieve thuough. Call it a lofty goal or an unachieveable ideal. Likely much easier on a newly installed unworn box, but still.
The actual test fitting of the fin can wear out the very box one is trying to get the fin to fit into and when test fitting it is so easy to go from too tight to too loose. I recommend wetsanding with 220+ grit once getting close. Fins I now make for my board will not fit into any other finbox without serious thinning of the base, and a store bought fin wobbles like mad and would need serious shimming on both sides and perhaps underside, to not wobble badly.
I have given up on the roll pin in favor of two screw tabs plates and screws and a straight insertion, with the screws merely to retain the fin, not leverage it into the base.
If you do go ahead, look into capping the sanded fin box with more fiberglass as this will help somewhat with the box stretching when test fitting, and over time.
When designing fins, I like using some crease free cardboard and some other fins to draw various size and shape fins, then cut them out stick them in the board and look at it from all directions.
Thats my CAD, cardboard aided design.