One thing about rolling your own: .1% of 0.1 ohm is 100 microohm. Obviously this is an order of magnitude less than contact resistance so you will need a 4-point kelvin connection. A commercial precision current sense resistor will have this done correctly. However, at this level spreading resistance and lead resistance are also significant. If you build a composite resistor from multiple lower power devices you will need to ensure you do so in a way that limits these sources of error. Of course, you can just measure the resistance with an accurate resistance meter, but if you have a sufficiently accurate ohm meter you can just take a 10W 1% resistor and measure that.