3Dlac is just a rebranded hairspray, presumably without perfumes added.There are a number of posts out there which particular brand of regular hairspray does the same trick. PETG may well stick to clean glass if you keep the temperature just below the glass transition point of PETG (around 70-75 degrees C).

I use borosilicate glass (heat resistant glass, the stuff that is used for wood burners etc)., but regular glass may work just as well. If it doesn't break, it is the cheapest option.

You can put it on top of the blue tape I guess, it may just slow the heating up time a teeny bit as it is an extra medium to transfer the heat through, but I expect you won't notice any difference.

If you put glass on top you will need to adjust for the extra height. I have a small thingamajig of 4mm I put under the z-endstop that I use to adjust for the 4mm thick glass in my FF creator Dual. My other printers I just adjusted the whole set as I found that I always print on glass now.