Bill's right. I want something to fit on the handlebar, and the rest of you are right too, whatever that fitting is must then provide a horizontal bar perpendicular to the centreline of the bike to mount another unit on, as in the examples on Bill and Dave's bikes. To answer Dan, you're right too: for a variety of reasons I don't want to disassemble the stem/n'lock/ahead-to-quill conversion, two of the most important being that I wouldn't be surprised if the quilt is stuck in there, and that my health is such that I can't bend over the bike for more than ten minutes at a time -- I ride better than I work on the bike! That assembly is solid as a rock, but I nearly gave myself a stroke getting it done to that standard. Also, now that I think of it, the stopper/locking assembly of the ahead gubbins with a ridged seatpost clamp (there's correspondence elsewhere on the forum about this if anyone doesn't grasp what I did) is relatively critical within a millimetre or two, and if any extension I fit in the place of a steering height spacer isn't exactly the same height as the spacer I take out, my bike could be at a standstill until I source the right size of adjusting spacer and wait for it to be delivered to darkest West Cork, just when the weather has turned (great ride to today, even if still in long underwear). That's not all: my bike's gear cables and brake hydraulic tubing are at maximum stretch and, while I have spares on hand because I intend recabling/retubing the whole thing and raising the handlebars another 5mm next winter, I don't want to be forced to do it now, because I just do not know how long it will take my fave bike off the road if working stints are only ten minutes long. (Don't even suggest I let the LBS do it. I like my life too much!)
Those are nice, tidy jobs, Bill and Dave. You don't say where you found the doublesided clamps, Dave. If I have to buy the one-sided Acor Bill uses, I think it likely that I will fit the barbag, as the heavier item, directly to the handlebars, and the pedelec computer to the Acor L-bar.
Thanks for helping out, gentlemen.