What kind of Slayer tone (i.e. from which song/album)?
Edit: I've tried making a patch before, the closest I got was using either one of the JMP-1's or the JCM800. I guess you could use a OD in front as said above or you could use a ten band GEQ with an upside down V shape like Kerry King supposedly does. Also using an IR with a G12T-75, G12K-100 or V30 should help. I thought a G12T-75 Marshall Mode Four IR with a Microtech Gefell UMT70S condenser mic from Ownhammer's Heavy Hitters Collection sounded closest, one of the first positions, either that or a M160 of the same speaker/IR collection.