Here are some pics of the new box boom I made. Because the original boom was too bendy and I have a loose-footed sail, I needed to make a stiffer one.
It's a box boom utilising clear 40X10mm radiata pine, and 19mm thick Paulownia infills. If you think of the GIS's gunwales, that is the basic construction method used. The front infill is 600mm and the very aft one is about 200mm. All the others are 70mm and 90mm apart. The longer front one is to give the boom greater strength to cope with the downhaul stresses. Even though it is quite light 3kg), it can support my 70kg weight OK, so it should be plenty strong enough. The top and bottom capping of the boom uses uber cheap finger jointed pine, but the sides are the more expensive solid pine. In one of the pics you can see the finger jointed stuff on the top. With all those buoyancy chambers, it should float pretty well!
If one used some decent spruce or oregon, I think it's possible to simplify the construction and leave the capping off entirely. This could make lashing the mainsheet and downhaul simpler, and they would not slide. I thought about leaving the capping off, but the sideways flex was more than what I wanted, so the capping was added to stiffen up the sideways bend.
Finished dims are 58X38mm with a 10mm taper along the bottom at each end. The taper at the front disappears over 500mm (half a metre) and the taper at the back disappears over 1000mm (1m). There is a slot cut at the back for a 30mm brass sheave which should make adjustment of the foot easier. I intend running a simple 1:1 control line about 1500mm from the aft end of the boom to a fairleaded clam cleat. There will also be a micro fairlead that feeds the line into the sheave on the top of the boom. In due course I'll post pics of the rigging which will explain things better. As they say, a picture is worth a thousand words!
The boom only ended up costing about $20 in timber. With the sheave, fairlead and clam cleat, total cost is a tad over $50.