I needed to create .txz manually, without the .Slackbuild file. I found a bunch of manuals for the old version, which was supposed to do this:
make install DESTDIR=/tmp/files And already inside this directory execute the command makepkg to create a package.
But this command did not help. The collected files were still copied directly to the working directories. Does anyone have experience? Maybe you can do .txz in another way?