Hi Presser...
Actually, in rereading your initial post I can see CLEARLY that you told us it was a cast iron pipe...
my mistake!!
Fact is that almost anything you install in this case will fail as roots have a tendency to find any moisture especially moisture with waste product in it and if there is a crack present... even the smallest of cracks... a root will find its way in there... grow ridiculously large and reclog the pipe. You would not believe what I have seen roots do or how far I have seen them travel to get to a pipe joint, enter through the joint.. A GOOD JOINT and then begin to grow out of control.
With that being said, everyone here will tell you that whatever we create... it probably won't keep roots out for very long. About the only thing I can think of that will last is a permanent patch over the hole using a liquid epoxy to install a patch over hole... but no extension coming off the pipe as it will shift back and forth over time and root (being present in area already) will invade quickly once again!
NOW, here is what I think... I think you should install a cleanout using the clamps posted by Milo by yourself! Easier than you might think if you have basic tools such as a shovel and a sawzall.
We talk alot of people through this stuff... in fact we have a 21 YO kid that just dug up part of his basement and installed all the pipes under a concrete slab for a bathroom.. click on this attachment to review his thread/posts if you like:
https://www.askmehelpdesk.com/plumbi...ml#post1233593
Scroll down to post #19 and post#25 at this link to get idea of how it works visually.. then read a little more of how it goes.
Come back after you review this and let us know if you care to try to install a cleanout yourself. We are glad to help here.
Otherwise... I think any solution you come up with will only be temporary... and I should know because I have seen maybe 30-35 cases where people tried to close off or patch pipes incorrectly underground and roots invaded every time!
Let us know what you think...
MARK