EZDIY-FAB Vertical GPU Mount with High-Speed PCIE 4.0 Riser Cable, Shield Series, 115mm/4.52in Flexible Extension, 90 Degree Connector, Only Compatible to Full-Opened PCIe Slots-Black-YIHPI344

Are you sure about that length? The FLP01 is only rated for a 309mm GPU. The 90 degree GPU mount that I used only added about 2mm of extra length to the GPU - literally the thickness of the sheet metal; the only issue was getting it into the case past that rim that you've already notched. It would probably be a tighter fit than mine; but if you willing to notch out more of the case, you might be able to drop the horizontal GPU in.