I have an inlay kit for a router that would do the job "IF" you are up to the task of operating a router on the ceiling. You don't need the power of a regular size router,I use a laminate trimmer 90% of the time. You could inset 1/4" plywood or any other material up to 1/2" thick to glue any shape plug with perfect fit every time. Look it up at a woodworking supply to see how it works.

Elmer's Glue company Wood filler will do it - you can get it to match the wood or just paint it with a craft acrylic that's the same color as the planks.

thank you to you both - I do not have a router. I'm going to cut a bunch of short slivers to get the matching saw dust and mix w glue to a paste and fill them, after I screw in a few wood screws to give it something to bite. Many of the knots are naturally darker so if I make it a bit darker than the pickets, it should work well. Does this sound ok?

I second the tinted epoxy. There isn't much that's more durable than epoxy.

If you want to fix this with a minimum of fussing around, and want to have to do this only once, get a wood epoxy restoration kit: Abatron kit

Here is a short video that shows you how this product works: Wood restoration

Tap plastics sells all sorts of epoxies and tints for the epoxy as well. I would want something that sets up fast.

you could also try cutting tree branches to fill the not holes . Epoxy them in and stain to match when done.

I realize that it may sound a little crazy, but its so crazy that it just might work.

happy filling

Kud & Dream, thanks for the suggestions...

I think you are both onto something w the epoxies. also, you can add a dry pigment to the epoxy to get the color right and forget the painting...thanks again and I'll send some pic's when done- tstex