First I spotted through the holes in the larger bush flange with a felt tip pen held vertically. Then I used a compass and rule to confirm the hole's exact location and centration before carefully center punching. Note the witness mark to place the slot correctly each time.
Once the holes were bored out to nearly the correct size I bolted the flange into place through two of the holes. Then ran the finish drill size through the flange holes to true everything up.
The bush was less than 2mm too wide to go easily into the channel. So I used an angle grinder to put two, small opposing flats on the largest flange. A file was used to take off the sharp edges. The smaller flange fitted with plenty of room to spare. The next image is a posed picture with a length of 50mm stainless steel shaft in place in the locking bush socket. No attempt has been made to torque up the temporary screws.
I think you will agree the arrangement makes a very neat and secure job. The bush will be protected to a large degree by the channel section saddle. With the tube rings mounted on the flat face of the saddle's channel section there will be a minimum of overhang beyond the end of the Dec shaft.
It seems I must try elsewhere for my stainless steel screws. 44 miles later and I have fetched the screws I needed on my touring trike. I found a large discount store in the city stocked the screws and load spreading washers. The even larger competitor next door had large, empty gaps in their stock display precisely where I might have had to rely on them.The first dealer only had M8 in 30mm which proved to be more useful than 25mm if I am going to plate over the saddle. The square washers I bought proved to be much too big to fit between adjacent screws. So my inability to obtain the original screws and larger, round washers proved to very useful.
The saddle looks smarter with a slight taper to the sides Here I have used galvanized washers but they aren't ideal and could be slightly larger with a smaller bore and preferably in stainless steel.
I am now leaning towards adding a top [and bottom?] plate to reinforce the channel section saddle material at the point where it is most likely to flex against the bush's flange. The ten screws and the face of the flange can do only so much with 5mm, 3/16" thick aluminium. Though any top projection on the saddle might limit clearance with the OTA. Steel has three times the stiffness of aluminium. So, where clearance is an issue, steel makes good sense.
However, unless I use stainless steel there will be a potential rust problem. Stainless steel is much harder to cut and drill with normal [amateur] equipment. Drills without a really sharp edge will tend to rub and work harden the steel. Tripling the thickness of aluminium plates will increase the stiffness at the bush flange. I could use an inverted length of channel section on top with the original with the webs cut down to allow OTA clearance. Or I could add a little packing under the tube rings if necessary. The channel section will not fit inside itself of course. So a simple plate would be necessary to go between the webs if I choose reinforcement internally.