Collapse the telescope down as far is goes and lay it on a piece of 1-inch foam. Draw a rectangle around the telescope with a marker and carpenter's square, leaving 1/2 inch of foam around the telescope, inside the rectangle. Trace around the telescope as well. Measure the height of the telescope while it is laying flat, using a tape measure.
Cut your foam rectangle out with a utility knife. Cut another rectangle of the same size for the top of the box, plus one for every inch of the telescope's height. Trace the outline of the telescope onto all but two of the foam rectangles and cut them out, using the utility knife.
Cut two pieces of 1/2-inch plywood the same size as the foam rectangle, using a table saw, for the top and bottom of the case. Cut two pieces as wide as the height of the telescope, plus 1 inch, and as long as the long edge of the top and bottom pieces, for the sides. Cut two pieces as wide as the sides and as long as the short dimension of the top and bottom, for the ends.
Apply glue along the edges of the top piece. Lay it flat on your work surface. Fit one end piece to one narrow end of the top panel, with its corners aligned to the corners of the top, and its bottom edge flush with the face of the top panel. Nail through the end piece into the edge of the top with three 1-1/4-inch pin nails. Attach the other end in the same way.
Apply glue to the upright edges of the two end pieces. Fit the long sides onto the box, with the ends flush with the outside faces of the end pieces and the bottom edge flush with the face of the top panel. Nail through them into the edge of the top and the two ends, one nail every 4 inches.
Apply glue along the edges of the remaining bottom panel and fit it into the open face of your box so that its outside face is flush with the edges of the side and end pieces. Nail it in place as you did the top.
Set the table saw fence 1 inch from the blade and set the blade as high as it will go. Start the saw and run the box over the blade, cutting one long side, with the top of the box against the fence. Install two 1-inch box hinges on this long side, bridging the cut, two inches in from each end. Attach the hinges with 1/2-inch wood screws in every hole.
Cut the two ends and the remaining long side on the table saw in the same way to create a 1-inch high lid. Position a hook latch in the center of the long side opposite the hinges, with the hook attached to the lid portion and the eye on the lower box portion. Use the screws that come with the latch to attach it.
Place one uncut foam piece in the bottom of the box, then stack the cut outline pieces in on top of it. Apply spray adhesive to the remaining uncut piece and fit it into the top of the box. Set the telescope into the cutout foam and close the lid, placing the hook of the latch in the eye to lock the lid closed.