Why not just make some kind of fire-proof housing for 2x S1's? Creating a solution from scratch for a small market geared towards home heating would defeat the purpose of using older, less efficient miners that otherwise would have been tossed or sold at a fraction of the purchase price. Otherwise it would just make sense to buy something like Spondoolies or S3's which would come in at the same price point (or less).
The ideal heat exchanger / radiator would allow you to easily swap out obsoleted boards for new ones. That way you could amortize the heater over multiple generations of mining hardware. Even better is enough room to keep older generation mining and just add new next generation boards so you can squeeze the last coins (and heat) out of old hardware.
All mining rigs are going to need the same thing - power, ethernet and cooling; the rest is just mounting. So in theory the radiator part can be fairly universal.