Installing a Cleanout Door

Hi all,
Recently we had a blower motor replaced for the gas furnace. Since then I have noticed a foul (oil) smell in the basement. May have been there intermittently all along. I have come to the conclusion that the cleanout door / chimney is the source of the odor and soon will be installing a chimney cleanout door in the basement. Do I need a special type of mortar? How close to the correct opening size do I need to be? Are the chimney bricks made of a special/different material?
What I have now is a door, not airtight, mounted on a wooden frame that is screwed into the mortar. The cleanout hole is a space were about 5 bricks were knocked out. I think that I will need to cut some bricks to size...maybe they come in half sizes? Do I need to worry about the mortar drying or setting to quickly?
I have not yet purchased the door...any suggested sources? (an online search turns up only two different doors from many sources) Will probably go to the local building supply store to buy, no HD for this one.
Thanks, Bill
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