Yeah, I used to have trouble with this MU, not as much anymore. My experiences are kind of like those of the other people here.
I haven't found mortar to be really useful in this MU. I think Fox's dair and nair(?) cancel it... so it's not a good approach blocker. Nades aren't really that useful either unless Fox is trying to utilt spam on Snake or is being stupid and wildly approaching with attacks. I like to use my c4 as I can since it kills Fox earlier than utilt. however most of the time I like be up close, abusing the range of ftilt (just don't get grabbed), or tech chasing if I can. Tech chasing is really good vs Fox. You can also shield grab quite a lot in this MU I find, since Fox's range isn't good. Pivot grabs are also really good, I use them a lot vs him. Stay out of the air! I've died so many times cause of this. Also, Fox's jumps are really high and super fast. Be careful in your recovery vs him. Most of the time a Fox will try to either uair or bair you out of cypher. Both kill really decently, especially bair. Watch out for that.
If the Fox does laser on the ground and I'm close enough I just run in for the grab/dash attack. the extra damage doesn't really matter to me cause he takes more than I do. Just throwing that out there.
In general, yeah, be pretty close. Don't try camping him back unless the player is really stupid... it won't work. You'll take more damage. Stay on the ground, use lots of ftilt, and lots of grabs. This MU is pretty close to even, if it's not even it's maybe 55:45 Snake but I'm not sure it's even Snake's advantage. They counter each other pretty well, imo.