If you have the boat on an open top or flat bed container if the cargo is over height. The carrier will charge you a bit less than if you had 2 containers (in your case 40ft) if your cargo is not over width. If is the cargo is over width then you will pay a similar charge as for 6x40 ft containers.
Lashing is important and I would recommend that you employ a proper stevedoring outfit to do this. Also insure the boat during carriage for damages (it costs Euro 60-100 depending on the value and it worth it!). Is the shippers responsibility for securing the cargo in the container and the carrier has no liability if anything happens to it.
IMHO DO NOT USE BROKERS (or FREIGHT FORWARDERS as they want to be called) such Peters & May etc. Go directly to the line (SEA-LAND comes to mind also ZIM Lines? Hapag Lloyd, P&O, MAERSK etc). Find which of these lines go that way and call them. You will save money and trouble (and I'm talking from my VERY recent unfortunate experience here). Also in case something going wrong you talk direct to the carrier rather than a broker who then has to go to the carrier who then has to come back to you (and they take loooong time in doing that as they d not wish to know).
The boat can be stood on V-shape cradles and if you have the boat with you they can be made in the local wood shop from hard wood made to measure. They cost nothing. Forget all this fancy staff of cradles as you will only use those once and probably they will be ruined during discharging of the boat and be thrown away. Unless of course someone else pays and money is not important