First of all, the internal mod NEEDS to be done no matter how big of a heatsink you get. The problem is that there are two thermal sticky pads that hold the ESC guts to the case. Those pads are fairly thick, do not conduct heat very well, and come unstuck after a while of running. For an external heatsink to work, you need to pull the heat from the FETs in the first place. Yeah, I know the mod is something that shouldn't need to be done, but we don't live in an ideal world so...
That said, adding caps is also a good idea. Since there isn't any extra room inside the case, you can add them on the power wires as close to the case as possible.
The easiest and most compact way to do this would be to:
- Determine the number of caps you want. 2 or 3 is good - any more won't help noticeably, but more won't hurt anything.
- Tie all the (-) lead of the caps together (denoted by the whitish stripe on one side of the cap) to make one thick lead. Do the same for the (+) lead.
- Strip about 1/8" thick of bare wire on both red and black wires as close to the case as possible.
- Wrap these leads around the stripped wires and solder. Make sure the black lead goes to the right capacitor lead!
- Ideally, using heatshrink on the soldered connections is preferred, but may be difficult. I used a piece of thick double-sided electrical tape.
- Insulate (heatshrink) the whole cap bank to totally cover the connections. It's best to leave 3/4 of the caps themselves uncovered to help them stay cool.
Just be aware that there will be a slightly larger spark when you hook up your batteries since the total capacitance is higher. This is normal.