Hi RoshKruv, Happy Christmas if you celebrate it
Before i go on, can you confirm you want to leave a 3-10W laser unnatended pointing into the sky? I cannot stress enough to you - this is a terrible idea and I would investigate alternative methods of locating your tent. I'm going to list a few of the dangers/Legal issues of this, and suggest a couple of alternative thoughts, and I hope you take my comments on board before you hurt yourself or others.
Powerful lasers are a serious eye hazard to people on the ground and in the air - if your laser ended up accidentally pointing at people at the festival, you could be responsible for the permanent blinding of many many people. At that power level, all it takes is a fraction of a second of exposure to burn a hole in someone's retina that'll never heal.
In addition to this, pointing lasers into the open sky is not something to be done lightly as airplanes and other air traffic can also be at risk from the same eye hazards as on the ground. A highly visible laser also reveals the location of the user very well, so expect visits from law enforcement or festival security damn quick if you're caught using something like this.
On a practical laser note - the most visible colour is green - and the most powerful diode available in this colour is rated for 1W - but this doesn't come cheap, as its usually around $150 for a bare diode, excluding any driver circuitry which you will need. Your voltage converter is no good unless it can operate in constant current mode and provide very clean power. A 1W NDG7475 will appear brighter than any single diode avaliable - a 7W NUBM44 for example will appear around 75% as bright as 1W of green.
My suggestion for you if you're set on a beam of vertical light is to look at a way of focusing a powerful LED flashlight to infinity in a narrow beam - but this is still really not a very good solution. Powerful LEDs and Lasers alike eat power and kick out a lot of heat - a diode like the NDG7475 will need a significant amount of heatsinking and possibly active cooling if you want it to be on constantly - and it's going to decimate pretty much any battery that you have to power it with. Is there not a GPS based solution you can utilise?
As an aside, the members of this forum are generally more inclined to help if new members post their rough location (helps us know what's legal in your country) interests and experience in the Welcome subforum