Making a lift duct the easy way!
This technique is (IMO) the simplest and easiest way to make a lift duct (short of having a mould!) with the least effort and time involved.
What you need:
Two sheets of foam the size of the finished duct lip, a few offcuts and a foam strip the depth and circumference of the duct wall. A modeling knife blade, a small length of wood the radius of the duct. A cheapo (as cheap as you can find!) ratchet strap. Resin, glass, filler, etc
Firstly, you make a couple of former discs from foam about 0.5mm or so larger than the finished duct internal diameter. The easiest way to cut the foam accurately in a circle is to glue a model knife blade into a cut in a length of timber at 90 degrees so that it sticks out 1/2" or so. Then bore a small hole to take a wood screw exactly 1/2 the diameter from the blade. You then screw this wood through the foam sheet you are cutting - into the table top and slowly rotate it around to cut a perfect circle (make sure the blade is at right angles to the foam surface). The centre circle will be used as a former and the outside section will become the duct lip/flange. Make two circles/lips like this.
Then stick the two circles together using a few equal sized spacers - for most lift ducts the final assembly needs to be a about 3" deep. To get the circles exactly above each other, lay one disc onto a flat surface and use a right angle square around the edge to line up the second disc.
Then take your strip of foam that is the width of the duct depth and at least the length of the duct circumference. Wrap this foam strip around the double disc assembly and trim it until the ends meet neatly - use the ratchet strap to pull the foam evenly around the disc (don't over tighten it or you will compress the foam). The outside edge of the foam should overlap one of the discs about half way (the other edge can be trimmed afterwards). If the foam pulls away from the formers around the ratchet or hooks then use small offcuts of foam between the strap and duct to apply more pressure in these areas. Once you are satisfied with the fit and there are no gaps between the former discs and the foam you can then saturate the nylon strap in resin and lay a strip of glass over it (stay away from the edges of the foam at this stage). Don't glue the ratchet mechanism or any hooks!
Once the resin has cured, trim the inner and outer edges of the foam strip and hot melt glue one of the lip sections so that it is about half way onto one of the circles and butts up against the duct tube - glue the second section on top of this one making sure it is aligned properly. You can then glass/resin the joint between the lip section and the duct tube. Once this has cured, carefully cut off the ratchet and hooks and lay one or two layers of glass over then entire outer duct tube and under lip.
When the resin has cured, you can form the duct lip radius by using a ply or plastic radiused scraper attached to the length of timber and rotating it around using the centre hole in the former disc (PU foam rubs away quite quickly). Then carefully remove the inner disc formers and glass the inside surface of the duct and over the lip onto the surround (you should fill any holes or gaps in the foam at this stage. A couple of thin layers are easier to use for this - try to make the finish as smooth as possible to cut down on final sanding.
Once finished, sand off any high spots in the inner duct surface and lip (you may need to fill in small imperfections. Don't bother sanding the outer duct surface or underneath the lip - it's never seen!
If you overlap some of the above procedures it's easily possible to complete a duct within about 1 1/2 hours.