Grab an Arisaka offset M-lok to picatinny mount. I find 1 'o clock or 4 o'clock works best so you can get a full arm extension thumb-over grip on the handguard without the flashlight obstructing your hand placement, then mount the tape switch on the top rail just behind your front irons (leave enough space for them to fold down if they're flip ups) so your thumb has full access to the on/momentary without overextending or breaking grip. This also keeps your lower rail free for any grips or hand/fingerstops you'd like to run.
I'd keep the light as far forward as possible to avoid casting a shadow from the handguard. Just keep it slightly rear of any porting on your flash hider/compensator/muzzle break, as putting it in the direct path of escaping gasses will foul up the glass quickly, as well as direct percussion putting it at risk of shattering.
Also, pick up a couple of these and clip out the middle segment to give you 2 sets of 2-rail wire clips from each one. They work really well for keeping slack out of the tape switch cable and keeping it tucked away and safe from snags.
https://www.opticsplanet.com/ergo-grip-lowpro-5-slot-picatinny-rail-wire-loom.html
