I'm basing my thoughts on something I saw in the Rifleman from the 40's or 50's when Krag were hot topics for sporterizing. To be sure, vertical spacing under the barrel is limited. That is why I like the simple strap, overlapped at the bottom, with a trapezoidal nut above it - the whole thing should take up no more than 9/16" or so. By inserting the block between the straight strap ends, and pinning it on the sides, a huge amount of valuable space seems to be wasted, at least to my eye.

The screw might have to be home-made. If you were going to fit QD swivels like Uncle Mike's, you'd need to drill a hole through the shaft of course. Or, perhaps such base screws are available commercially. You also need a shoulder to draw against the escutcheon (es-CUTCH-en). I suspect some such have a wood thread which is fine for a modern forend, but no good for us. Screw length would have to be figured very carefully, probably by trial and error, as you need to be hard against the 1/8" inch or so escutcheon, fully engaged in the nut, yet not touching the barrel.

As to the swivel bar, without a jig and mills, you'd have to be very careful; it would be tough to get it right - that is why, when you have doubts about a "carbine" (talking trapdoors here mainly) one of the first things you look at is the bar inletting. Were I to do it, I'd first locate the hole centers VERY carefully, drill them (slightly undersize) with an old-fashioned brace and bit, then finish the cut VERY slowly with chisels and scrapers. I don't know any other way to do it.

This has been a great discussion!