Flip the assembly. Being careful that the placed t-nuts do no fall out.

Take the remaining 4 t-nuts, which you previously tested, and slide them into the extrusions as shown in the image.

Tech tip: You can tape the t-nuts to the centre of the extrusions. This will prevent them sliding out of the extrusions during the following steps. (two small pieces of Blu Tack also work)

Do not place tape on the outer, 20mm, faces of the extrusions. The sides will be placed flat on the table and the thickness of the tape could affect accuracy whilst squaring the frame.