Most trainers use profiles that are similar to Clark-Y. Often the profiles are modified to have a absolutely flat bottom which Clark Y doesn't have (even though it is close to flat bottom). There are a zillion profiles to choose from but I suggest that you have look at the NACA 24xx series airfoils, e.g. NACA 2415. These airfoils are not flat bottom but if you put spars at about the thickest point of the airfoil and make the bottom side flat behind the spars it will be very simple to construct the wing.
The 24xx series have been used for a lot of model aircraft with very good results, nice stalling behaviour, fairly low drag etcetera.

BTW, don't forget to include a couple of degrees of wash-out when building the wing (2-3 deg is enough). It's almost an overkill if the planform is rectangular, but if you choose to build with wash-out you can be shure not to have any nasty tip stalling tendencies.

(Washout = outer portion of the wing has a lower angle of incidence than the inner portion.)

/Red B.