You could get a PCIe switch backplane, stick your GPUs on there and then have them communicate locally through the switch using p2p. For some GPUs you may have to use a hacked driver (google p2p 3090 for instance).
A good alternative is ADT, they make excellent boards, there is a 4 and a 5 slot PCIe expander with a 88096 on it that works extremely well. I have two of these connected to my rig with their own power supply and four GPUs in each (and another two in the main machine). It's not exactly a portable affair (to put it mildly). Note that these won't work in a standard PC case due to the slot spacing so you'll have to rig something for that yourself (I use 2020 + some custom 3D printed fixtures).