Have two thimbles, need to plug the upper one, how should I?

Hi, I have an oil fired trailer furnace in my workshop. It goes into a block chimney through a 6" thimble through the wall. Below this is an 8" thimble that runs through the wall. Both run through a brick section of wall. I am going to remove the oil furnace, and put in a wood stove. I would prefer to run the wood stove through the 8" opening, how should I plug the 6" opening? It worries me a bit since the 6" one is the upper one.

brick and mortar

Ok, any special type of brick? I have high temp mortar already. So I can put the new 'plug' that I'll be building on the end of the thimble next to the room? I found this 'insul plug' product out there, but no way I'm spending $100+ on a plug if I can just block it with brick+mortar.

Fire brick and your hi temp mortar should do it...

Should I make an effort to build the plug near the rear of the thimble? Or does that not matter so much? Obviously that would be a bit more difficult.

yaa...make an effort dude