Thanks for the suggestion! I've already done that, and there is no spacer specifically for the P-40. I also asked Kel-Tec as to whether the 15 round magazine spacer would work with the 11 round 4006 magazine, and they didn't know. In the meantime, I've ordered the Plus One magazine adapter for the P-40, and may just stick with that. Given that the modified factory magazine will now give me 10 rounds in a compact format, the 4006 magazine with 11 rounds is dramatically larger, and sticks out of the grip further than the equivalent 12 round magazine does in the P-11. At this point, I think that I've just talked myself into returning the 4006 magazines.
FWIW,
emc