The only problem you have now is the ratio of R1 and R3. I see you made it huge to get an approximation to a square wave. That only works because your frequency is so low, and you can make those resistors high. But if you want to scale to a higher frequency, R3 will become too small. A better way to make a square wave is to make those resistors the same value, and use a diode across R1 anode connect to the junction of R1 and R3. That allows the cap to be charged thru R3 only, and discharged thru R1.