I would favor mechanical connections vs. chemical.
As I read your brief, your objective is a large donut nominally 26" outside diameter, with body of 1 1/2", less turning allowances; say 1 3/8" body and 25 7/8" final OD.
On similar work, I've made the first ring of segments about 1" or so wider than its brothers. Attach with screws in this waste area to a plywood disk, radius slightly less than the mid-point of the donut body, e.g. 24" OD in your case.
Turn the outside profile of the donut, to the edge of the plywood disk on one side, and full width on the other.
Attach several (more = better) bent metal brackets to the disk to reach past the outside of the donut, with cushioning and spacers to assist centering. Wrap the brackets with a very large hose clamp assembled from many smaller ones, to clamp the work. Flip the piece to cut off the wider ring and shape that region of the donut. Flip again to shape the region on the other side.
An interesting alternate to the wider ring for the first turning would be to make the disk flush with the inside ring. Place screws in the interface so that they engage both the disk and the ring, half in each. This is an extremely robust connection, sometimes used in large steam engines and pressure vessels. The final turning would remove the "footprints" of the screw threads, and might be applicable if you've already cut your segments. In fact, upon reflection, the first turning could shape all except that quadrant of the donut body.
And upon further reflection, the first turning of the other process could also incorporate its third step.