There are plenty of FETs in large packages like the one you found - if you have some spare gold bars to pay for them . I did some detailed research and concluded that unless you need to switch thousands of amps, FETs in a smaller package like the TO-247 has the best combination of current capability, heat transfer and size. I picked the IRFP4004 FET and mounted four of them to a small heatsink with copper strips to connect the source and drain pins. Its fairly compact and will comfortably switch 120A continuously in a real-world situation - more if I increased the heatsink or used a fan.

If you want to DIY a single direction FET switch,have a look at the OSMC circuit: http://www.robotpower.com/downloads/...2sch-clean.pdf. you just need to duplicate one of the low-side FET legs and then find a suitable driver. The HIP driver chip isn't necessary as you are only driving one string of FETs and you don't need the voltage pump as all the FETs are on the low side of the circuit.