Active load with mosfet and op-amp
The devices you have do not have a suitable DC SOA. The device is rated only for pulse operation at high powers. Going beyond 1 ms on time puts you in a very dark place. 

You need to use something like an IXTH30N60L2 ...these devices are rated for large linear load dissipation.

Read this on the L2 range from IXYS.

For the devices you chose, SOA is this: 


You actually need something like this:


....for your application you could use just two devices with plenty of margin or select a larger one and use just a single device.