I have built numerous speed controllers using wheel chair pwm and have purchased PWM circuits from the robot store. Either circuit can be rigged for speed control using a 5k potentiometer. I have built several and waterproofed them. I put the pwm circuit in an old car stereo amp box and used the amp box for the heat sink and board container. Got the amp box at a yard sale. I sealed the unit using RTV silicone. Power in and power out was pigtails...I made custom end plates from sheet aluminum.

Ebay has some pwm circuits (china) but I have never use them. I have used 4QD circuits in the past.