When I was making my turtle planters, I found the best way to get good tights joints in the octagonal body was to make a jig for my miter saw so that all like parts had the identical length. I needed different widths and three different lengths, but the basic jig would be the same. I made a base that bolted to the saw table with a stop at one end so it would be repeatable and then three different stops that were located by dowels to assure repeatability. For a one of project - I've made 8 of these planters to date, and contemplating another run of three - you would just need a simple table with one fixed stop.
Showing the original (clamped) stop block and the three different lengths of segments required - although each layer is a different width, the inside dimensions are the same so that the cavity where the flower pot goes is the same dimension top to bottom.
Showing one of the removable stops (located by dowels) in place & making the initial cut on the end of the board. Not shown in the photo, but there is a stop underneath the plywood table on the right that locates against the end of the saw table so that the fixture always locates to the same saw kerf.
Showing the wood flipped front to back and the cut end placed against the stop, ready to cut to length. To make multiple parts, continue to do this until the board becomes too short. Repeat until you have the required number of pieces. I was starting with 8' lengths, so there is an outboard support for the boards to the right of the saw.
Showing an assembled body, with the five layers glued and screwed together - the body is actually an oval hexagon - the consistent lengths of the parts went a long way to giving me nice tight joints, really struggled with the project until I took the time out to make the fixture. Now, if I damage one of the segments, I can easily set up the fixture and make just one piece that's identical to those made previously.