I successfully created my own. Roughly from memory I must have
tce-load -i java-installer
tce-load -i squashfs-tools
mkdir java8-installer
cp -R /tmp/tcloop/java-installer/* java8-installer
... Edited file replacing 7 with 8 in just a few places. I also updated the UPD default.
mksquashfs java8-installer /mnt/sda1/tce/optional/java8-installer.tcz
The last line assumes that your persisted extensions are mounted on /mnt/sda1 --