> Your only option there is to destroy a Monopoly or After Burner.

Not sure what you mean. krikzz sells a Master Everdrive if you want something suitable for multiple games. If you want a single game cartridge for a homebrew release, raphnet has you covered.

The one exception is if you have a Japanese Master System (or Mark III). These have a different cartridge connector that's shared with the SG-1000 and SC-3000 and flash carts are harder to find. You may be able to use a western SMS flash cart with a passive adapter though

I’ve already got an Everdrive. I like making single game carts. I was unaware of the raphnet product but that’s considerably more expensive than original carts.

There's also this open source design: https://gitlab.com/doragasu/sms-sl2map

It doesn't handle all the features of the standard mapper so not compatible with all games, but it's designed to be cheap to manufacture. Unfortunately I don't think anyone is selling these so you'll need to take the files to a PCBA house (or get bare PCBs manufactured and do assembly yourself)

Sir, I am a few days belated but thank you so much for linking this. I feel terrible roaching copies of After Burner.