Here is the blower I used for the firepit. Its a SEAFLOW 320 cfm 12v flange mount marine bilge blower for boat ventilation. None of that matters other then voltage, volume of air, and mounting. They can be had off of eBay for relativity inexpensive. They have different mounting models, but i chose the flange mount as it was the easiest for me to work with.

I cut a little piece of pipe off of the thick walled piece that will be buried under the fire part of the pit and welded on a flange. The flange was created by tracing the front of the fan onto cardboard to make a template. Found a piece of sheet metal that was thick enough and big enough and cut it out from that.

Welded side of the flange. Had to be careful to to make the weld too thick around the mounting holes as it would then interfere with the mounting hardware.

I cut a gasket out of gasket making material. Now this isn't needed at all but I figured I would do it to give a little cushion between the fan and the pipe in case of any abnormalities in the plastic or metal itself. Like a burr or molding seam line that might cause undo stress and break something if it was hard surface on hard surface.

Once all that was ready to go, I just found some mounting hardware that fit and bolted the parts together.

The next thing to make would be the final adapter that fits the greenish blue drain pipe that runs to the pit. The metal pipe flange i made has an outer diameter wider then the drain pipe, and the inner diameter is the same as the drain pipe. So to make an adapter, I first started with a drain pipe coupler and cut the length down a little. Then carefully with a handheld torch, I proceeded to carefully heat up one end of the coupler until the plastic started to get soft. When it seemed ready, I would take the metal flange and push it into the plastic coupler causing it to stretch around the metal part until it fit. This took a few times to get it done as the metal pipe would cool down the plastic pretty quick. I didn't want to heat the metal as I needed the plastic to cool and set.

With a fully finally formed plastic coupler, I pushed it onto the metal part one last time. Now all i have to do is slip it on the end of the drain pipe and have an easily removable blower that has a pretty good seal on the pipe.

Looking down the outlet of the fan with everything mounted. I got everything perfectly aligned so no turbulence in the air flow. Not that really maters, I just like to make things as best as I can.