4.5" tall Gusset corners are not available from 80/20 vendors, but you can get raw stock of the profile and machine your own using these 3D Solid or Pdf files!
.step / .x_t/ .pdf blueprint Files will be available for download after payment.
.step & .x_t files are intended for use in 3D software. Few other programs will even be able to open them
I do not currently have physical parts to offer, but I am exploring options to provide them. In the mean time, if you don't have access to a CNC, you could get the files quoted from a turnkey supplier such as ProtoLabs or Xometry.