Phill on this list may be better to reply as he has just co-designed the "Blade"..

For a light platform one below the proposed class spec.. You have two paths..

1) Tortured ply.. As Carl suggested check out "Gougeon Brothers on Boat Construction".. Also study the Tiapan 4.9 web site especially their tips for home building.. Dont forget Phill's home page.. Ply Tiapans when well built are very competative.. As Bill stated in a previous post
Yet they were designed 3-4 decades ago.. Hell they arent even tortured ply but stitch and glue chined ply design.. Yet as Phill also stated they are built to under 150 lb..

2)composite.. Check out the UK Cherub website (male mold) and Javelins.org and their Virtual Javelin project.. Not sure but also look up the International Moth web sites (The Aussie one used to have building tips).. Yes some Moths are still built in ply.. With perhaps a weight inrease from 10 kg up 17 kg hull weight..

I know of a home built International 14 that was built over a winter by three guys.. Working every second weekend.. 1 cm foam over a male mold.. 200 gm twill carbon outer skin with a second layer around the plate (knee damage when righting).. Inner skin was 100 gm twill.. The false deck was similar but the whole upper skin was 2X 200 gm and a mate layer.. With a complicated (and heavy) system to rake the centreplate from the trap.. All fittings ect.. It was 10 Kg under the class limit..