I got the top of the cabinet scribed in and cut to fit well enough… it’s not perfect, but good enough for caulk to fix the gaps.
Then me and Ennie tried to place the dominoed-together top (dry fit, thankfully) onto the base.
It got wedged in the wall and tore up a chunk of the drywall.
Off to Home Depot to pick up supplies and fix it. Sigh.
Thankfully I know how to deal with this… thanks to YouTube mainly. Heh.