you could combine some aerogel and some XPS (extruded polystyrene). I used vacuum panels cored with aerogel sourced from Glacier Bay (claimed R50). The build process was a huge pain in the butt and extremely expensive, and you have to have at least some XPS outside of it anyway because of the edge losses. If you can get in 4" of XPS on a frig (6" on a freezer) you have reached the point of diminishing returns. My frig has about 36 hour holdover which is nice, but I'm not sure I would do it that way again.