I used to sail mini 40 tri's, I was also the secretary of the British Model Multihull Association. I can help with tips, but the main one if you are building from scratch is to keep it light.
My last boat was a mix of Andy McCullocks latest main hull, Freight Train, and french floats from the club at Cergy Pontoise in France. The boat was about 4lb all up weight and came third in the last Coupe De France I entered. I also used swing rigs, originally the boat was conventional rig, but it was re-rigged to swing rig when I fitted the French floats. I make sails, (
www.windpowersails.8m.com) and particularly used to specialise in Multihull sails. Though now In do have patterns for most classes.
You also need to fit a wing on the bottom of the rudder, typically a spitfire shaped wing with plenty of depth, this can be set at zero angle of incidence and will stop the boat from nose diving and tripping, this is another reason the boat needs to be light, so that it accelerates rather than tripping over the lee float.
If you make the wing to slim, you will not get the same down force (the wing is upside down) and will need to set a couple of degrees down into it which will cause drag. The wing can be made adjustable so you can slip acouple of washers in it to alter the angle, but if the shape is correct this will not be required.
More info can be found at
http://www.apolloboatservices.co.uk/Anthony is not related, just a similar name. GBR6 is my second tri.
The British model multihull assoc
http://freespace.virgin.net/model.multihulls/If you need any other info let me know, you can send a pm or through the forum, my contact email is at my sails website.
Regards Andy