Mine was Not HDPE plastic. You will never glue that together. Nothing sticks to HDPE.
Mine was built of 3/4in Marine plywood.....Epoxy glued together and then covered in fiberglass mat.
You take the footprint drawing to the fabricator that will work in alum. Have him build a Hallow box out of 1/4in alum.....The oval tube that the rode runs thru is built out of a piece of alum tube....cut in half.....then pieces of flat welded between where it was cut.....to form a Oval as in the drawing. It would be a good idea to have tubes welded inside where the unit is thru bolted. This will make for a VERY strong piece.
Make the unit about 8in or 9in tall.
Now go hunt a source for fabrication in your area.