Build it yourself parts required:
MBC3 chip (take from an MBC3 cart)
MBC3 8 pin power management chip (take from an MBC3 cart)
4MB MBM29F033C Flash
32KB FM18W08 FRAM
CR2032 Battery holder + CR2032 battery
32.768 KHz crystal (take from an MBC3 cart)
300K resistor 0603 (take from an MBC3 cart)
2x 20pF capacitor 0603 (take from an MBC3 cart)
1K resistor (take from an MBC3 cart)
10K 0603 resistor
10K 0805 resistor
4x 0.1uF 0603
ROM: 4MB Flash (MBM29F033C)
FRAM: 32KB (Data retention of 20-50 years)
MBC: MBC3 original chip
The power consumption of MBC3 when powered from the battery appears to be around 2.3uA so a CR2032 should last around 7-10years.
The PCB finish is ENIG, Gold fingers (on the edge connector) with a 45 degree bevel to give the maximum life for insertion/removal of the cartridge.
If you will be installing this flash cart in an original shell, it is very difficult due to the CR2032 battery holder but it is possible.