More a case of make a repair than make something new. About ten years ago I made an oak picnic table out of offcuts left over from sculptural pieces made for an arboretum. The sculptures came out of what were basically green oak sleepers that were rough sawn at the mill to anything from about 6" X 6" and up to maybe 8" X 8". Getting enough material for the design was a bit tight and some of the bits were borderline, specifically there were some inconveniently placed shakes, knots, and sapwood. I just went ahead anyway and used what was there rather than spend extra money on replacement wood. Anyway, one of the table top slats contained a fair chunk of sapwood and, as is to be expected, rot set in and the sappy edge crumbled off. So a repair was called for.
Below is the table when it was new.
Below shows the repair from the underside along with the rotted slat. I cut out the old slat with a multitool using a couple of crosscuts. Then I machined up a bit of replacement oak, and cut a tenon at either end. At the top of the picture below can be seen how I got the tenon in at that end by routing out a stopped rebate to the depth of the outer cheek of the mortice in the thicker outer frame, plus a piece of replacement wood to make up the thickness of the frame. Waterproof PVA was slapped around, the slat slipped into the mortice at the bottom of the picture and the far end dropped into place and the infill replacement piece of wood clamped and screwed in.
And from above the new slat looks completely out of place with the old slats, see below. This non-match colour wise of old and new doesn't bother me because in a year or so all the parts will probably just about blend together. As a side note, apart from brushing on a bit of linseed oil when the table was new just to make it a bit prettier for a portfolio photograph at top it has never received any other finish (paint, varnish, oil) since. It had its first ever clean, i.e., scrubbing off dirt, mildew, algae, or whatever with a wire brush and plenty of water earlier this year, and the table sits out unprotected in all conditions all year. I'm one of those that prefers, I suppose, in careless or benign neglect of outdoor pieces of furniture, i.e., very rare maintenance, no varnish, no paint, etc, but I do pick a decently durable wood species to build with. So, not a very exciting or difficult project, maybe three hours work, but just a wee task I felt I ought to do before the broken rotted slat irritated me too much, ha, ha. Slainte.