Gather these parts

Glue the larger frame onto the non-detailed side of the backer plate

locate the hex plastic sheet

Place smaller frame over the hex sheet and trace the outline with a pencil

With a razor score the outline you traced in the previous step in the center of the line between hexes as shown.

Flex the plastic along the score line and it should snap out fairly easily

Flip the hex plastic so that the convex parts are down and glue on top of the smaller frame you used to trace previously

Glue the frame with the plastic into the void in the first frame on the backer plate as shown

Glue side struts in place as shown