At first I thought I’d have to get out the calipers to measure the various dimensions, but turned out there was a nicer solution – the datasheet already has drawings of the chip, drawn to scale. So getting the sewing pattern just becomes a matter of tracing out the datasheet drawings in Inkscape:
The various traced out parts are then scaled to the desired size, and borders then added for the seams. The chip is padded out with a rectangular foam block, and folded-up strips of aluminium foil are used to strengthen the legs.
The next natural improvement would be to actually embed a real OP-07 within, and have conductive threads sewn into the various legs. But in the absence of conductive threads at home, this is the best that can be done for now.
An interesting observation: If you play with this long enough in a dry room, and then pick up a real OP-07, the latter throws a hissy fit (out of jealousy, no doubt) and releases all its magic smoke.