For V4 scope, you just need a glass window, no lens implantation for L2/3. Don't need to open dura. Just make sure to put on very thin layer of dental cement so the scope can get to the window closely so you can access more neurons. You need high quality cranionomy window surgery.
For V3 scope, depends on the lens combination and working distance. I used to use a combination that I implant the objective lens against dura (press down ~200 um) . That is what I used to collect most data. Later I found out that the lens combination in this product (http://www.newdoon.com/en/index.php?route=product/product&product_id=60
) can give you ~ 500 um working distance so you can also use it to image through glass window, the objective lens stays with the scope and outside of the window.
In my hand, keep the dura intact is better, the window will be clearer.
I don't have experience with L5/6 neurons.