Yeah, a split ring washer there is probably not doing much. Possibly even making things worse. Given it's more or less impossible to sufficiently torque a wing

Yeah, a split ring washer there is probably not doing much. Possibly even making things worse. Given it's more or less impossible to sufficiently torque a wing nut, there are actually not many great solutions... A dab of low strength loctite would definitely help, but that's one-time-only and you've got to carry the loctite around if you want to re-do the connection... At which point why not just carry a spanner instead and just use a properly-torqued normal nut? Probably the best would be to swap out the plain wingnuts for nylock wing nuts, which should at least help a bit.