Don't.
Ferro is making a comeback. I'd estimate about 70% of my builds mount it right now. Simply put, many times you can get away without running XL by using Ferro, ending with a far more durable build.
The key, and I cannot stress this enough, is that this primarily works on builds where you aren't mounting equipment in your arms, or only one arm. That's why Torso-only builds are absolutely brutal and are among the best in the game; a good example would be the Centurion 9A's typical 2ML and 3 SRM6. This is made possible by combining Endo + Ferro.
Ferro has a reputation for being underpowered, but in cases like that - that means a much bigger engine with absolutely no downside. Ferro is a very powerful piece of equipment to add to 'mechs running Standard Engines, and I highly recommend you look into ditching the XL in favor of Ferro on any builds you can manage it!
Finally, Ferro is very friendly towards large engines. A 'mech with a 375 engine can mount all of it's extra heatsinks right inside, freeing a huge amount of slots; in turn, the Ferro will help provide the weight necessary to mount such a large engine and the DHS within. There are already some killer projected Battlemaster builds that utilize this fact.
Edited by Victor Morson, 19 September 2013 - 01:53 PM.