I've successfully used the MIPS mosfet block (product code MCH1163 on http://www.mips-computer.de/) for the R3G. However, the block separately does NOT come with a backplate for the mosfets on the back of the board.

Don't know if they include a backplate when you buy the whole set.

For the NB I've used the HEATKILLER NSB Rev3.0 Ni. You need to trim the mounting plate on one side because the inductors on the left of the NB interfere with it, but apart from that it fits fine.

I presume the R3G blocks for the NB from MIPS will fit fine too.

Also bear in mind that the blocks from MIPS DO NOT come with any thermal pads. You have to buy those separately.