Its hard to calibrate for calculating actual amperes, but you could just measure voltage over some piece of cabling you already have in your car.

So for example, measure voltage drop from alternator to battery, if there is clearly one wire and no other current paths that might take some portion of the load. If you need to get real ampere values instead of just a blind graph of differences, you probably need to get DC-current clamp for reference. I purchased “UNI-T UT210E” and have been quite pleased with it.

Another option, somewhat better/easier, but more expensive, is to buy DC-clamp with voltage output. AC would be easy and cheap, but DC is quite complicated.

I have a fluke i1010 amp clamp for ac/dc current measurements on things over 60 amps and I use a PDI CA-60 for up to 60 amps. Either of these would work with the Mooshimeter. The Fluke would have to be adapted but the PDI plugs right in. Unless you want to check amp output driving down the road which can be done but you can get cheap amp clamp multimeter combo for just testing alt amp output while under the hood.

If you need absolute measurement you’ll need a calibrated shunt, something like this:
250 uohm Current Sense Resistor
200 Amps through a 250 micro-ohm shunt will give you 50mV across the shunt.

Otherwise you could just use a length of thick wire with suitably heavy terminals. But it’s difficult to calibrate.

It’s not that difficult to calibrate a DIY wire shunt. You can’t easily measure the resistance, but you can calculate it if you know the gauge and composition of the wire. Search for an ohms per foot wire chart online, and choose an appropriate length for the wire. Caution, stranded and solid will have different values.