/diy/ - Do It Yourself

[Anonymous No.1668703]

Hi /diy/,
I want to install a folding door for a shower, but I have a problem: the two walls where the door is supposed to be are not parallel; in fact, only one wall is perpendicular to the pavement, while the other one is crooked in two dimensions (pic related).
Now the company where I ordered the door proposed me to buy an aluminium profile for 60€ (a technician came to my house and took measurements for free to ensure I would buy the correctly dimensioned door), but this profile is supposed to correct only the "front" slope, not the "top" one (pic related). This means that basically the profile would have to be annoyingly adjusted in order to make it parallel to the other wall (company's technician's words). The guy who proposed the profile told me stuff like "put some bolts behind it as spacers in order to make it parallel", even thought this would leave some awkward spaces between the profile and the wall.
Point is, I'd like to make a corrective "slab" of marble myself, by buying some marble from the local equivalent of craigslist, cutting it with the right angle (I'll use a circular saw with a diamond blade) and fixing it on my wall.
My question is: what am I supposed to use to "glue" some pieces of marble to a tiled wall? Keeping in mind that it has to be water resistant since it's a shower. I never did these kind of jobs, I see there's a lot of "grouts", "mortars" but this is all new to me and it's the kind of job that doesn't really allow mistakes.
This is what I was watching as of now:
https://www.amazon.it/gp/product/B0013IQY12/ref=ox_sc_act_title_2?smid=A11IL2PNWYJU7H&psc=1