Generic rootfs:
http://resources.crux-arm.nu/releases/3.5/crux-arm-rootfs-3.5-wifi.tar.xz
http://resources.crux-arm.nu/releases/3.5/crux-arm-rootfs-3.5-wifi.tar.xz.md5

Cross-compile toolchain:
https://releases.linaro.org/components/toolchain/binaries/latest-7/aarch64-linux-gnu/gcc-linaro-7.5.0-2019.12-x86_64_aarch64-linux-gnu.tar.xz

Installed the toolchain to /opt/toolchains.
Added /opt/toolchains/gcc-linaro-7.5.0-2019.12-x86_64_aarch64-linux-gnu/bin to $PATH.

Cross-compiled the kernel:
mkdir -p target/boot
git clone -b rpi-4.19.y --depth 1 https://github.com/raspberrypi/linux rpi4-kernel
cd rpi4-kernel
make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- bcm2711_defconfig
make -j$(nproc) ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- Image modules dtbs
make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- INSTALL_MOD_PATH=~/rpi4/target modules_install
cp -v arch/arm64/boot/Image ../target/boot/kernel8.img
cp -v arch/arm64/boot/dts/broadcom/bcm2711-rpi-4-b.dtb ../target/boot/
cp -av arch/arm64/boot/dts/overlays/ ../target/boot/
cp -v Module.symvers System.map ../target/boot/

uSD card formatted like so:
Disk /dev/sdb: 29.83 GiB, 32010928128 bytes, 62521344 sectors
Disk model: microSD RDR
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x565d8b1c

Device     Boot  Start      End  Sectors  Size Id Type
/dev/sdb1  *      2048   264191   262144  128M  c W95 FAT32 (LBA)
/dev/sdb2       264192 62521343 62257152 29.7G 83 Linux

Created and mounted filesystems:
sudo mkfs.vfat -F 32 /dev/sdb1
sudo mkfs.ext4 -L / -m 1 /dev/sdb2
sudo mount /dev/sdb2 /media/usb
sudo mkdir /media/usb/boot
sudo mount /dev/sdb1 /media/usb/boot

Uncompressed the generic rootfs to the uSD card:
sudo bsdtar -C /media/usb -xf crux-arm-rootfs-3.5-wifi.tar.xz

Cloned the firmware repo:
git clone --depth 1 https://github.com/raspberrypi/firmware
sudo cp -r firmware/boot/* /media/usb/boot

Copied the kernel, modules, dtb:
sudo cp -av target/* /media/usb/

Added cmdline.txt and config.txt:
$ cat /media/usb/boot/config.txt
arm_64bit=1
disable_overscan=1
dtoverlay=vc4-fkms-v3d
dtparam=audio=on
gpu_mem=128
hdmi_drive=2

$ cat /media/usb/boot/cmdline.txt
root=/dev/mmcblk0p2 rootfstype=ext4 rootwait quiet

Edited /media/usb/etc/{fstab,rc.conf} appropriately