The circuits are designed to handle all kind of fault conditions. Shorting the inputs or outputs to any voltage between 0 and 12V will not damage the circuits. The supply voltage is 5V, but if you would apply 12V, nothing will go bad in these circuits. Although the circuits here are fault tolerant, I do not know if the MAFs or ECUs are. The ECU can handle voltages between 0 and 5V for sure, but I'm not sure it will not be damaged by voltages above that.
The conversion is quite accurate as you can see in the measurements below. It's best to use 2% resistors. You can use 5% resistors, but you may want to check the actual resistance value. Normally 5% resistors are within a few percent, so good enough.
Low cost components
The components used are all very low cost. 0.25W resistors and standard small signal transistors. You can find these at any electronics shop at a cost of something like one dollar in total for the parts. I used BC547/BC557 transistors, but many small signal transistor will do. (2N2222/2N2907 is okay too) Don't waste your time making a beautiful printed circuit board. Just use an experimental PCB. That really is good enough. The circuits are not critical on PCB layout at all.
For the casing it is important that it is water tight. If not, the circuit will corrode and will go bad in no time. The circuits are so small however that it will fit in a 35mm film container. Just punch small holes in it for the wires and seal it with silicon. The container can be placed next to the MAF.
You are free to use the information presented here. I do not ask any money for it, but also do not accept any liability. You are not allowed to sell the schematics or printed circuit boards with these converters with a profit. If you find a circuit valuable and are going to use it, I would be happy to hear your experience with it.