Reddit mentions of Josi Minea x 2 Pcs 2.5mm Male to 3.5mm Female Audio Adapter Converter Headphone Earphone Headset 3 Ring Jack - Stereo or Mono [ 2 Pack ]

Checkout the VModa BoomPro on Amazon for $30 bucks new ($22 renewed). I'd say it's on par if not a little better than the modmic for the price.

Modding your M40s to fit a 3.5mm jack would make it work. (I did it myself)

Here is the video I used.

Here are mine
Here is a GIF of mine

Here is the adapter from Amazon

The V-MODA Boompro has a 3.5mm male connection, whereas the HD598CS has a 2.5mm female connection. Unless you modify the headphones with a 3.5mm input (which is relatively easy), the BoomPro will not be compatible.

Alternatively, you could get this microphone, which is similar to the BoomPro but has a 2.5mm connection

or

You could get something like this and plug it into your headphones, then plug a V-MODA BoomPro into the adapter.

There's a 2 pack right here... https://www.amazon.com/dp/B011S90B6E/ref=cm_sw_r_cp_apa_2HoSBbEZ1T264