A butt joint like that is never gonna be invisible with a bright finish, the color & texture just doesn't match what's along both sides.

I'd settle for "smooth as glass" once a final finish has been applied. Means forcing in some thickened epoxy now so the gap is both filled and sealed well, then careful sanding & maybe another coat or two of clear, unthickened stuff before varnishing begins.

As spclark says, it'll never be invisible but use thickened epoxy to fill the gap. If you use sawdust from the same bit of plywood then it'll blend in well.

Invisibility may be in the eye of the beholder.  <;-)

.....Michael

   @ Yambo. I tried that on a test piece. Came out noticeably darker. 
thanks though

Hi Pappy - that's true, 100% woodflour with epoxy will be darker than the wood the flour came from.  But you can mix in some cab-o-sil to lighten it up and provide a decent color match.  This isn't a great pic, but you can see the fillets on my Skerry are a decent (certainly not perfect) color match to the wood.  

Hopefully I've learned how to post pics without breaking anything!

Remember the woodworker's motto: Don't hide it - Highlight it!

I had a similar issue on my Chesapeake 17.  The deck butt joints near the cockpit and the deck scarf joint near the bow turned out pretty ugly.  I bought a veneer inlay strip in a pattern that I liked, and inlayed it over the joint.  The ugly joint became a distinctive design feature.

https://www.google.com/search?q=veneer+inlay+banding+strips&sxsrf=ALeKk00V774RXFxAmNltlxFYvA1cVkq1dA:1591490175381&source=lnms&tbm=isch&sa=X&ved=2ahUKEwi4qMT0uu7pAhUbCc0KHSKpAIsQ_AUoAnoECA0QBA&biw=1920&bih=942

i think you are overthinking it.

looking at your picture, you did very well.   

another approach that i use on strip builts is just some regular sandable wood putty of a matching color  for example: https://www.acehardware.com/departments/paint-and-supplies/patch-and-repair/wood-filler/1538024?x429=true&gclid=EAIaIQobChMI6J2ivMLu6QIVj4TICh3YrQ5wEAQYAiABEgLfsfD_BwE&gclsrc=aw.ds or i will make a little of my own with wood glue and sawdust.

lay it in, let it dry then sand it.

i find this helps keep things lighter than wood dust in epoxy and is easier to control.  it's also fine because you are going to epoxy/glass over it which ensures that everything stays waterproof....this is eally just a cosmetic fix and is not about addressing a structural issue (e.g., its not a fillet)  where epoxy would be expected to be used.

h

   So I painstakingly fit a piece of okoume into the joint. I think after a darker colored stain I'm considering and glassing it will pretty much disappear. 
Thanks for all your help

http://www.imagevenue.com/ME123O84