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A few "bearings" are inserted along the bottom of the rim, the axle inserted, & the remaining "bearings" slid in around the axle circumference.
Bearings are retained via a large SS washer on each side. All are kept in place by a larger delrin washer. All that retains the washers is a cotter pin through the axle.
From the factory, holes are pre-drilled to accommodate 8' or 8'6" boats.
You could make the delrin washers out of almost any stock, just drill holes for the cotter pins as needed. FS sells sets of 4 SS washers for $6. You would have a hard time finding cheaper.