I 100% back up Skunkie in buying a real deal mono cart. I’ve also got an AT33mono, which sounds better than a stereo / mono switch or Y cables to sum the channels together.  As Skunkie pointed out,, there’s a lot of “mono” carts available, but most are just internally summing the R+ and L+ outputs.

Before buying an AT33mono cart, I made my own “mono” cart by taking a stereo cart and wrapping a small thin wire around the R+ and L+ outputs of a stereo cart. While not a necessity, I put another wire between the R- and L-. I’ve found a figure 8 type pattern works best.

Then, just put the lead wires on the cartridge. By doing so, you’re simply summing the two channels together early in the chain, but just externally instead of internally. While it still won’t be a true mono cart, it can give you a taste of what a true mono cart can do.