I can't recall ever seeing the H207 stack used in a MM11-E show up on eBay. Maybe even less likely than finding a complete MM11-E!
Plessey made memory units for various PDP-11 models. I suspect that you'll have better luck looking for a DEC MM11-L or Plessey (or perhaps another third-party like DATARAM) equivalent. DEC memory might be listed as a ME11-L, which was a separate 3U (9-slot) subsystem holding up to three MM11-L. I have seen MM11-L show up on eBay, possibly listed as some combination of G110/G231.
There are two PM-1116B (16Kw) listed on eBay at the moment (
https://www.ebay.com/itm/315063367621 and
https://www.ebay.com/itm/226001813404) that should get you much closer to your goal. The complete manual, with schematics, was recently recovered (by Roland?). It states "The PM-1116B may be operated with, or in place of, the DEC model MM11-L in applications where -15V at 6 amps is provided." and "The PM-1116B memory module requires the same DC voltages as the DEC MM11-L memory." The H720 is spec'd for 10A on the -15Vdc rail so I expect that you'd be good for one of these. Hosting two would be ideal, and that might actually work with the H720 as-is.
The PDP-11/20 was capable of supporting a _pair_ of MM11-E. While an individual MM11-E stack was 8A worst-case active at -15Vdc (2.5A at +5Vdc) only one stack would be active at a time. The PM-1116B was only 5.3A worst-case active at -15Vdc, so a pair of stacks should work fine on the -15Vdc rail. Note however that the worst-case requirement on the PM-1116B at +5Vdc is 3.3A (0.8A higher) so that rail might become problematic, even with a single stack, depending on what other modules you have installed. You'll want to check your power budget ...
There are options for solving the backplane/SU part of the puzzle if you decide to go down the Plessey path. I may be able to help in that case.