if you can fit any extra foam in your boot you got the wrong size....For a size 12 in a skate shoe you should be in about a 28 boot....I'll put money down that you are actually an 11 in shoe size and could fit in a 27.5 ski boot.
My suggestion is to start with the right sized ski boot.
29 and 29.5 are the same size....The difference between half sizes in ski boots is the liner...It is "prepacked" or 1 mm thinner then those found in a full size.
I'd recommend getting your boots fit by a professional boot fitter, you have the wrong size boot. Im a 12 in some shoes for the most part im an 11. I wear a 27 in my full tilt t'walls. I used to fit boots. Yours are two big 100%, if you can fit anything extra in your boot they are to big, no denying it sorry bro but you fucked up in the store when buying your boots.