Has anybody made their own?

Im not spending Â¬Â£85 on one and Ive tried to get one from Halfords but no joy. Been looking on fleabay at 12v pumps but dont really know what to look for. Found some wit a flow rate 10 times that of the dirtworker but only 1/6 the pressure but I am assuming this is directly out of the 12mm pipe and using a nossle from say a garden weed sprayer will bring the volume right down and raise the presure quite a bit. But will it be enough?

Are there any **SIMPLE** calculations I can do to help me work it out?