Once you have calculated the value you require for your shunt, it is not too hard to assemble an accurate resistor of this value. The important thing to remember is that resistors are cheap. Even 10W wirewound resistors cost maybe $0.15 each if you buy bulk.
If you want a 0.01Ω resistor for a 100A shunt, it will dissipate 100W. (Multiply the resistance by the square of the peak current.) If you use 10 0.1Ω resistors in parallel, this will give you the required value. If you use 10W resistors, this will also give you the required power dissipation. (Beware, they will get hot.)
If you want a 0.001Ω resistor for a 100A shunt, it will dissipate 10W. If you use 100 0.1Ω resistors in parallel, you can use the smallest and cheapest resistors you can buy. Because of the price mechanics of bulk purchase, they will cost maybe $0.64 for all of them.
The accuracy of the shunt you construct will be the same as the accuracy of the chosen resistors, provided they are all the same. Nowadays 1% tolerance resistors are nothing special.
The biggest problem is the effort of soldering them all up.