You need a spacer to run it in reverse assembly. Many things can be used, ranging from a simple battery fuse, to magnet spacers, to even two stacked pennies. Just make damn sure if you're using something improvised like magnets or two stacked pennies, that you are also using o-rings or something to insulate the outside circumference of them from causing an auto-fire by sliding and making contact with the sides. Most spacers from hybrid kits also work.
If you do a search for Hammer clone, there is a very long but informative
thread.
Outside of my main-squeeze Reos, a Hammer with an Origen on top is still the only other mech I use. Everything else is just another tube with some random engraving on it boasting great voltage drop numbers for cloud chasers IMO. With a little tweeking, the Hammer is highly modular, and the form factor can't be beat. Nothing is easier to set down and pick back up without a worry, not even my Reos.
Plus, it doesn't look like a dildo.
Cheers