I built an inline ballast awhile ago, It worked pretty well. I bought one of those " build it yourself 1000w hps" ballasts and configured it inside a long rectangular box ( had it custom made and welded by friend ) had the outside spray foamed and had 6 inch ducting attached at both ends and had 4 rubber coated D rings on the top and attaced in at the very beggining of my ducting and then through by lights and it hung from the ceiling with rubber straped..NOTE IF YOU HANG A BALLAST FROM THE CEILING MAKE DAMN SURE ITS ON BEAM!!!!!!!!! it worked extremely well then sold it off when i stopped using 1000s and went to 600s.