12 volt dc circuit help

I need to produce a 12 volt dc square wave ?pwm
From the square wave I need to be able to alternate the 12 volts dc from -+ to +- to the load
This would need to be to be adjustable from say from a second to 15 seconds give or take
It needs to be able to withstand max 30 amps dc
I will also have a cooling fan for the electronics
Im looking for a very easy build and cheapest but be able to last
Any ideas on the circuit i would need I would prefer to pay someone to build the first one Id buy it from you. It would be alot easier for me to copy that way then to make from scratch
I would need a few of these Thanx

It sounds like you need an H Bridge. What is the load?

This is what im doing I have a hho generator that Im running test on
It draws about 7 amps I fuse the circuit at 15 amp I wanna have it heavy duty to be able to handle up to 30amps i wont be using for that
I just want square wave dc and be able to flash reverse polarity + - to - + speeding up or slowing down can be many times a second to say once every 15 seconds Id need to adjust that manually