https://www.murrays.com/product/13-1409/All it is is a piece of oversized piece of aluminum tube that slips over the existing axle and allows you to expand or contract the axle as needed. Cut the original axle to fit the narrower boat. Then slip this pipe over one of the axle halves and thru bolt it in place. The other side gets two holes, one for each boat width and you use a bolt or clevis pin to lock the axle in whichever position you need.
We used this when we had a H18 and H20 (two different width boats) and it worked fine.
sm