Uses a 10k resistor and Quad Analog Switch IC 4066 DIP14, schematic by Trimesh and following info:

Normally when you plug a cart in it grounds pin B32 of the cart connector - this is wired to the ASIC and when it’s grounded the TMSS ROM gets paged in. So if you just cut this line it disables TMSS - but it also disables the ROM CS/ line fed to the cartridge ROM, so the game won’t run. To get around this, ROM CS/ (pin B17) is disconnected from the ASIC and wired directly to A23 on the CPU address bus. This maps the ROM into the lower half of the CPU address space, At this point, the ASIC still doesn’t know the cart is plugged in, but the CPU can see valid instructions and will run the cart anyway.

This is obviously a very simple modification (2 trace cuts and one piece of wire), and it does effectively disable TMSS - but it also breaks compatibility with the various Mega Drive/Genesis addons. The 32X and the power base converter won’t work at all, and the Sega CD will work, but you can’t use cartridges when it’s plugged in (and in fact run the risk of damaging the cart, Sega CD or both through bus contention).

You can fix the 32X and PBC compatibility by checking the M3/ line (pin B30) - if it’s low, you have an accessory connected and you should renable CART IN/ and connect ROM CS/ back to the ASIC output pin. From a quick look at the schematic, this seems to be exactly what the mod in the first post is doing.

Personally, I would be inclined to use an analog switch chip to handle this, since it would be a lot simpler

Try this - note that it hasn’t been tested and might not work, but it looks reasonable, and it’s just a single chip..