I turned an inner and outer cone to mould fiberglass cups for sealing off the holes where the rear of the plenum meets the engine mounts. The cone is 95mm in diameter reducing down to about 70mm in diameter at the flat-bottom. The flange is about 20mm wide, which will give plenty of room for trimming and for plate nuts.
I made the surfaces as smooth as possible and used plenty of mold release wax and PVA agent. I wet out three layers of woven cloth and a few doubler strips for the flange, layed them up and sandwiched everything together under weight. Everything looked good in the dry run, so I have high hopes for the finished product.
I'll trim the excess and put the plug in the lathe chuck to trim the edge round before I break open the mold. I pre-drilled a 10mm hole in the plug to help remove it with a lever.
