Hi Rainsend, 2" thick would be pleanty for concrete blocks. We made some with a purchased mold for making path/patio blocks and it is only 1 7/8" thick. We used an integral color and they've held up fine. They've been through 3 or maybe 4 winters in zone 5. I had planned to put a concrete sealer on them but never got around to it. We did have some that crumbled but they are near the driveway and were exposed to salt( concrete's enemy). However they are the same blocks that were made with a questionable bag of cement and I suspect that is what caused the problem. I can't recall exactly how much we figured the savings to be - about 1/3 the cost of purchasing even the cheap ones - I think. And seeing as you already have the sand it should be a good bit cheaper. However the time factor is another matter. It's definately a labor intensive project. We did a 12' x 12' patio and a 50'path. It took about 2-3 weeks but DH and I did it together and we have a cement mixer. If you would be mixing by hand I suspect there's a good possibility getting sick of it about halfway through. I also have 1 rhubarb leaf stepping stone but it's only been out this past winter - so far so good.

I know Connecticut Stone in Milford, exit 40 off of I-95 will do this. I forgot how much it was but it seemed reasonable to me. Call them up!

You can use either natural or cultured stone to put on the water trough. They do have Pre fabercated panels of both or loose pieces as well. They have many different thickness's and widths. Make sure you use a medium bodied stone thin set and water proof your brown coat you put behind the stone and the inside of the trough. Hope this helps.

There are steel caps for HVAC pipes, and I assume they would fit your flue pipe. If the cap was popped on, attached with screws, and also sealed with heat resistant caulk, that should be adequate to close it off. This link from HD shows several different sizes of metal caps. https://www.homedepot.com/b/Heating-Venting-Cooling-HVAC-Parts-Accessories-Ducting-Venting-Duct-Accessories-Caps/N-5yc1vZc5h9  This caulk should be acceptable. https://www.homedepot.com/p/3M-10-1-fl-oz-Red-Fire-Barrier-Sealant-Caulk-CP-25WB-Plus-CP25WB-10/100166701 Bruce