Installing Perko switch
 

I'm adding a second battery and Perko switch, but have one minor issue.

The power cable coming from the engine has a fitting designed to be attached to the battery post, but the back of the Perko switch requires a much smaller "eyelet" fitting. Is there an adapter/fitting to make this work, or do I have to cut the battery cable and make my own?

Chop if off and install a ring terminal. You'll have another half dozen to do so you may as well get the tools.

Put that end back on the battery, pull the other on off the engine (i'm assuming it's attached to the starter) and run it to the B1 terminal of the switch. Then go to the auto parts store and get battery cable of the correct length and gauge that has a ring terminal on both ends. Run it from the COMM post on the switch to where ever the original cable was attached. You will also need one for the ground and for the POS to B2 of the switch.

TigeMike - There is only one cable running from the engine to the battery area (that is the cable in the picture). Are you saying it's not OK to cut that cable and put a ring terminal on and attach to the COMM post of the switch??

I've already got both batteries tied into the switch on B1 and B2 with separate red cables. I've also already got B1 grounded to the engine, and B2 grounded to B1.

I just realized after posting what I need to do (I think), and this may be exactly what you are recommending.

I need to get a battery cable with eyelets on both end, and hook that to B1 along with the cable pictured above, and then connect the other end of that cable to the COMM switch, correct?

Ok, sounds like you are further along with the install then it sounded from your OP. If you have the 2 batteries tied together and a positive cable running from each battery the the B1 and B2 posts of the switch and connecting the COMM post to the original cable is all that's left, then by all means, chop the battery terminal off and solder/crimp on an eyelet. That will work just fine, no worries.