My son's bike has a 1x10 drivetrain with an inexpensive Suntour Zeron crank (bought new), new KMC chain and Sram X7 rear derailleur. The bike was tuned up by a LBS so all is in good working order. The only problem is that the chain falls off very often. There is no bash guard on the chainring and no chain guide. The crankset is a direct type so there is no spider to mount some sort or bashguard. Any recommendation for an inexpensive chain guide that we could use? I guess a frame mounted one.