Railway traction motor-sized SCR's!
I did something like this some years ago and I was suprised by how cheap really big semiconductors were. Don't forget you can use a standard Motor driving Contactor if you prefer instead of a semiconductor. There's no need for a heatsink either.
When you're choosing a semiconductor, just buy big, it's not that much more expensive.
Tip: If you want a feelgood glow about the strength of your design, make sure you understand the Transient Thermal Impedance curves in the datasheet.