A script for those who "Forgot to make a SlackBuild". Following a manual install (e.g. `make install`), use this script to make a Slackware package.

f2sb.sh

#!/bin/sh -eu
#
# This script allows you to create a Slackware package from software you
# previously installed manually (e.g. via `make install`).
#
# Usage:
#
#    ./script name-version /path/to/reference/file [seconds]
#
# Provide the name and version of the package, path to one file that forms
# part of the package, and optionally seconds either side of that reference
# file's creation time. If the third argument is not provided, 10 is assumed
# because unlike compilation, installation happens within a few seconds. On
# completion a Slackware package will be created.
#
# Example:
#
#    ./script zstd-1.3.7 /usr/local/bin/zstd
#
if [ "$USER" != "root" ]; then
  echo "Run this script as root" >&2
  exit 1
fi
NAMEVER="$1"
CTIME=$(stat -c%Z "$2")
RANGE="${3:-10}"
ARCH="${ARCH:-$(uname -m)}"
case "$ARCH" in
  arm*) ARCH=arm ;;
esac
PKG="$(mktemp -td f2sb.XXXXXX)"
mkdir "$PKG/pkg"
cd "$PKG/pkg"
find /etc /opt /usr -newerct @$(($CTIME-$RANGE)) ! -newerct @$(($CTIME+$RANGE)) -print0 | cpio --quiet -p0md .
/sbin/makepkg -l y -c n "${OUTPUT:-/tmp}/$NAMEVER-$ARCH-${BUILD:-1}.${PKGTYPE:-tgz}"
cd - >/dev/null
rm -r "$PKG"

x-f2sb.sh

#!/bin/sh -eu
#
# Alternate version that does not use makepkg and hence does not require
# root.
#
# NOTE: *This script could fail to find files in directories it has no
# permission to read!*
#
# This script allows you to create a Slackware package from software you
# previously installed manually (e.g. via `make install`).
#
# Usage:
#
#    ./script name-version /path/to/reference/file [seconds]
#
# Provide the name and version of the package, path to one file that forms
# part of the package, and optionally seconds either side of that reference
# file's creation time. If the third argument is not provided, 10 is assumed
# because unlike compilation, installation happens within a few seconds. On
# completion a Slackware package will be created.
#
# Example:
#
#    ./script zstd-1.3.7 /usr/local/bin/zstd
#
NAMEVER="$1"
CTIME=$(stat -c%Z "$2")
RANGE="${3:-10}"
ARCH="${ARCH:-$(uname -m)}"
case "$ARCH" in
  arm*) ARCH=arm ;;
esac
PKG="$(mktemp -td f2sb.XXXXXX)"
mkdir "$PKG/pkg"
cd "$PKG/pkg"
find /etc /opt /usr -newerct @$(($CTIME-$RANGE)) ! -newerct @$(($CTIME+$RANGE)) -print0 2>/dev/null | cpio --quiet -p0md .
mkpkg() {
  if [ "$1" = "-n" ]; then
    TAROWNER=""
    shift 1
  else
    TAROWNER="--group 0 --owner 0"
  fi
  if find * -type l | grep -qm1 .; then
    mkdir -p install
    find * -type l -printf '( cd %h ; rm -rf %f )\n( cd %h ; ln -sf %l %f )\n' -delete > install/symlinks
    if [ -f "install/doinst.sh" ]; then
      printf '\n' | cat - install/doinst.sh >> install/symlinks
    fi
    mv install/symlinks install/doinst.sh
  fi
  case "$1" in
    *tbz)
      if command -v lbzip2 >/dev/null 2>&1; then
        cmp=lbzip2
      else
        cmp=bzip2
      fi
      ;;
    *tgz)
      if command -v pigz >/dev/null 2>&1; then
        cmp=pigz
      else
        cmp=gzip
      fi
      ;;
    *tlz) cmp=lzma ;;
    *txz) cmp="xz -T0" ;;
    *) echo "Unknown compression type" >&2 ; exit 1 ;;
  esac
  if [ -x /bin/tar-1.13 ]; then
    tar-1.13 $TAROWNER -cvvf- . | $cmp > "$1"
  else
    tar cvvf - . --format gnu --xform 'sx^\./\(.\)x\1x' --show-stored-names $TAROWNER | $cmp > "$1"
  fi
  echo "Slackware package \"$1\" created."
}
mkpkg "${OUTPUT:-/tmp}/$NAMEVER-$ARCH-${BUILD:-1}.${PKGTYPE:-tgz}"
cd - >/dev/null
rm -r "$PKG"

Based on ideas that are explained in more detail in this gist.