You're not gonna be able to fit much of anything in an Antec PSU
I built my own relay using the instructions I posted earlier.
I first opened up the PSU and soldered two leads onto the main connection.
I then made a hole in the PSU case so the wires can get out and such...
Then I put a relay into a box and wired it up.
The same can be down with the Swiftech Relay kit. It is simple.