How to fill the holes in cinder blocks

[Prairie Valley Bread]

Does anybody have any brilliant ideas on how to fill the holes in the cinder block so the cement doesn't just fall through before pouring the top?

Also, has anyone used ceramic fibre (or fire) board under the fire brick for
extra insulation?
I'd appreciate any help.
Cheers,
Tom

[Les]

Re: How to fill the holes in cinder blocks

Does anybody have any brilliant ideas on how to fill the holes in the cinder block so the cement doesn't just fall through before pouring the top?

Those of us that used bagged concrete just wadded up the bags and stuffed it into the holes. Could probably use newspaper as well. You should fill every few cores solid to lock in the block, every core is not necessary.

Also, has anyone used ceramic fibre (or fire) board under the fire brick for
extra insulation?

Not sure by what you mean "extra insulation" 2 inch board should be used at a minimum.