When you say "smaller than your typical baseboard radiator", I'm not sure in exactly which dimension you mean? The
Runtal UF2-24 isn't any tinier than a 2' section of Slant Fin, though it might look a bit nicer.
Replacing a 2' fin-tube with a 2' cast-iron baseboard takes about the same space with comparable BTU output, but it's
not super cheap when bought new, but not Runtal-priced either.
But this sort of stuff shows up on
Craigslist all the time, at ~$15-25/foot.
They clean up and repaint easily. If you're not the DIY type you can probably find an auto body shop willing to do a bake-on automotive white (or some even nicer color) on the cheap next time they're doing a car-job and it'll look BETTER than new stuff, and still come in well below new cost on cast iron baseboard.
If going with used cast-iron, make sure it's not a middle section, but a standalone, with the cast-in leg on each end, with pipe threads on each end. It comes from a few manufacturers, at a few different heights too, so pay attention to the dimensions.
The
tiniest flat Buderus might fit your space (or not), not super cheap, but they come with a thermostatic mixing valve to fine-tune the output.