It's a victorian house and the boiler condensate goes from the neutralizer straight in the sewer line which unfortunately is cast iron so have to either keep changing it every month or come up with a bigger neutralizer kit.
Just an idea, but you might find a used plastic softener brine tank pretty cheap, or even free. I would look for softeners on CL. https://hudsonvalley.craigslist.org/for/d/water-softener/7047898157.html
Some may be free and include a brine tank. Some want more than they are going to get, and might reduce prices. They are kinda big... about 15 to 18 inches square or round and maybe 40 inches tall typically. There is a plastic overflow fitting, and a tube that goes to the bottom. I don't know if you would have to add 1/4 cup of bleach every 6 months or so to keep stuff from growing.
I am thinking to run the condensate to the bottom, and let the tank overflow out of the overflow fitting, which is barbed to hold a tube, eventually. I think you could fill that with a lot more than a bag, but I don't foresee a problem with only the bottom inch or two being calcite. I have no experience with this. If the output overflow clogged, it would still take a long time for the condensate to rise to the rim. I am guessing maybe 30 or 40 gallons to get to the overflow and half again to get to the rim.
Another idea is to use the resin tank of an old softener to hold the calcite instead. Official calcite tanks have a fill port separate from the valve. However you could just unscrew the control and fill from up top every few years I suspect. So the control could actually be failed, and still do this job. An unpainted (tan) tank would be best, because you can backlight it with a bright flashlight to see the calcite level. If using an old softener, you would dump the resin. That is not hazardous.
Not as compact and pretty as the official system.