On the one we did they gave spare rings. I guess they only gave you two?
I think they are needed. They are all that keeps the tanks from moving. A sealant probably would allow movement.
You should be able to buy just the rings if you need to.
On the one I did one hole appeared to be oval but that was just because the tube was cut at an angle. When I slid the PVC tool over it it fit pretty well.
Try keep the ring as straight as possible over the tube. If it's getting cockeyed then hit the pvc tool on the high side. I didn't hit the PVC directly with a hammer, I used a board over it.
For the one that's cut at an angle - put the pvc tool over the tube first. Note the angle the pvc is at. Try to start the ring at the same angle.
If you give it a couple of wacks with a hammer and you don't seem to be making progress double check to see if ring is straight. When it's off it seems to require a little more effort to get back to straight.