Doc, you don't want anything between the MOSFET case and the heat sink/spreader other than a thermal compound to fill in the air pockets.
Every layer offers another opportunity for micro air pockets to form, causing a huge increase in the thermal resistance. The foils won't help to transfer heat any better than having the case of the MOSFET directly contact the heat sink/spreader....even if it was diamond. No matter what the substance, it still has some thermal resistance and you'd be adding that thermal resistance to the ones already there (junction-case, case-to-sink, sink-to-air).
Take out the copper foil, leave out the silver foil, just use a good compound and (I completely agree with Jeremy) separate the bar into independent electrical sections. You've now got a huge spreader than can efficiently pull the heat from the MOSFETs. Do not use that Loctite 3873 if you're separating the bar into sections! That 5mil gap that the compound forms is a HUGE gap and is much. much worse than using a good thermal transfer (unfilled) compound like Aavid UltraStick or Wakefield 126.
To electrically insulate the bar sections from the case, you can use any good insulating (filled) compound or a good Sil-Pad or split-Mica. Do not use any foil between the bars and case either! No matter how conductive it is, it has more thermal resistance than using nothing at all. Since split mica (and good thermal transfer compound (unfilled!!!) on both sides can be a lot thinner than that 5mil gap for the Loctite 3873 epoxy, try the mica if you can. Otherwise, check the thermal conductivity ratings for the other gap fillers like SIl-Pad. I remember some having ratings up to 1.6W/m·K. That's useless if it's 20mils thick, but if the they're thin enough, they might be worth using (in sheet form between the bars and case).
IIRC, you used a dremel polishing wheel to smooth the copper spreader. While a smooth surface is great for enhancing thermal conductivity, the small size of the wheel can easily cause the polishing to make the surface uneven. That is, the surface is smooth, but not flat. I strongly urge you to get a sheet of 600 grit (wet) sandpaper, place it on a very flat table or other surface, and place the MOSFET mounting surface of the bar down on the sandpaper. With gentle pressure, move the bar in a circular motion just a few times and then look at the mounting surface. If the abrasion of the sandpaper hasn't covered 100% of the bar's surface (that is, you can see lots of untouched spots), then your bar isn't flat. And this can be a lot worse than a bar that isn't smooth.
A flat, but "rough" bar still has thousands of metal points touching the back of the MOSFET as it is pressed down. The thermal grease then fills in the low spots. Not ideal, but not bad.
A bar that is not flat, but is mirror smooth, causes entire sections of the MOSFET to hang in midair with absolutely no direct touching to the heat sink. Even though the thermal compound fills in the big air pockets (even a 1mil deep pocket is huge), this can be a much worse situation than the rough flat bar.
Ideally, you want smooth and flat. If the sandpaper test shows an uneven surface, press down the entire bar firmly and evenly and use a circular motion to continue to wear down the surface until it's even across the entire bar. Then switch to an even finer paper and "polish" the surface (all at once, face down on a smooth, flat surface) until it's even again. Wipe clean and it's about as good as you can get it. Polishing it further, by hand or machine, would just make the surface uneven again.
Also, and very importantly, all of this is for naught if the case can't efficiently remove the heat generated by the MOSFETs. You have to make absolutely sure that enough heat is dumped to the ambient air to prevent the MOSFETs from overheating. That case is a huge "thermal wall" and will, by far, be the biggest problem in removing the heat. Even if the case feels cool, it can be a big problem. In fact, the case feeling hot can be a great sign. It can mean than the MOSFETs are efficiently moving their heat the case instead of sitting there burning up.
