Not really clear what you want to do nor what the purpose for it is " Green laser gate/grid, as a portal, between two rooms, " that nobody is going to walk through or have their eyes exposed to potentially dmageing level of laser light? Is there any real purpose for such a thing?
If you want to do what is what the videos show--a serious quality effect-- you will need to fabricate long bars with as many lasers as you want lines either just up and down or crisscrossed to make squares---you say "Size of the grid 4x3m 13x9Ft " " Must be well visible, with light on, or at least suffused, no fog/smoke " so lasers strong enough for beam to be seen with light on---
For 1 laser every foot would need 22 lasers and 44 if every 6" if that is what you mean + you need a way to align them all perfectly so they are pointing in same direction in same plane and making equidistant lines.---figure $100 to $150 each for the diode, drivers and module + a power source or sources + whatever tools, mounting , aligning, and fixture costs---not a low cost project to make a credible grid of the type you want. Could easily become very expensive.
If it could be easily and cheaply done it would have been very many times over and efforts of others would be available to copy
What about where the beams terminate?--Will be a lot of very bright spots lighting up the area with a green glow.
Suggest you buy a 300mW green laser with active cooling--fan, TEC or both, a 12 volt power source for it, and a diffraction grid and experiment with it to see what it does and if what it does suits your purpose in the real world before you take on something like making multiple laser bars
For $80 to $200 --several choices here:
https://www.ebay.com/itm/Focusable-...424074?hash=item43f9c7b6ca:g:rKAAAOSwImRYOCq7
If using a hand held pointer, consider how long do you want to leave whatever it is you put together "ON" .
Keep in mid you have to consider duty cycle, tine on vs time off, to avoid damage and/or burning out the laser, of any laser you decide to use.
For instance a Thor M2 1.6W 445nm blue pointer can only be left on for a minute or so, 2 min max before you have to turn it off and let it cool completely.
If you want to leave it on for extended periods of time look for a laser with a continuous duty cycle , good for running continuously.
You can have a look at this 3 projector laser grid "ghostbuster" low cost device for $169 and see if it suits you here:
https://www.theghosthunterstore.com/product-p/puckgrid.htm
