There isn't a farm for Daily missions Prototypes weapons, the best way to get Custom Prototype weapons from Slot Machine is just rng, so just farm as much money from missions that gives a lot of money.

Normally the Daily Missions on Insane gives you roughly minimum of $10k per run, alway quit the mission before the last score so you can save 30 seconds of your time.

Once you get $999,999 this is where the fun begins

What I normally do is make a copy of your save file

Press Window Key + R and type " %localappdata%low/Sky9 Games/ "

this will take into your save file of the game you find a bunch of save files like
SaveGame_Backup.sfh from 1 to 9 and SaveGame.sfh

Always copy the " SaveGame.sfh " file somewhere you can find that is your save file of your game. You can override the save file if you replace the new save file with your old one.

If you don't get the Prototype weapon after $100,000 or don't get it at all, you can reset with the copy save file you have and reset so you can get all of your money back.

This is the best way to farm good weapons of you need better weapons.