The way I designed the tank heater is probably only feasible on the 300d sedan due to the orientation of the fuel tank. If you want to design your own, go for it.. pretty much anything you can do to heat the fuel pickup area is going to be beneficial. Use a cheap range hose like I did, or have a custom pickup made.. the important parts are:

1. Make sure there is no possible way the coolant can leak into the tank
2. Make a good seal, so the grease doesn't leak out of the tank
3. Maximize the amount of hot surface area exposed to the cold grease
4. Maximize coolant flow through the system

There are so many variables involved when trying to measure how fast the tank heats up, such as how full it is, ambient temperature, type of driving I'm doing, etc, so I've never gotten a really good measurement on it. I will say though that after driving around town for a few hours, the whole tank is rather warm to the touch and the whole trunk is emitting heat. So I think I did a pretty good job, especially since it cost around $15 (for the hose and a tube of JB weld). :-)