Sorry, I assumed you would be installing a clay liner....and I know you are, You can go with square or round flue tiles, and the opening I was talking about would be in the side of the chimney, not the vertical portion. The thimble block have semi circle already prepared. You can not buy them already prepared, but it is worth the extra buck or two to have the things done. Remember to saw-cut the clean-out hole and flue hole into the flues before you assemble the thing.
You can try to create the openings by hand without sawing, but you might need to purchase a couple extra flues for that adventure. If they call you "lucky", please disregard.
Building Code here says that you must use refractory cement between the flue ends as you ascend toward the top of the chimney. It is not cheap (50 bucks a bucket- enough for a 25' chimney), but the masonry cement will not withstand constant heat (wood fired device, coal doesn't matter, as you know) and acidity of oil fired appliances. We both know that there are millions of chimneys without refractory between the flue ends, but I do it now, and feel safer for doing it. Don't forget about wall ties to anchor the chimney to the building surface, and include a couple large heavy duty straps lagged into an overlay or rafter as you go up.
I was sure you were using an 8" flue in the 16 square chimney block.......