Buy some thin flexible cutting boards made from PTFE, or silicon baking mats in a kitchen store. Cut to size, then just a tad smaller than the winch
base. Use a hole punch for the bolt positions (buy one if needed, well worth the small expense) that is one size bigger diameter than the bolts. Winch
manufacturers (or the mast
makers? can't remember...) sell these insulating gaskets ready-to-go too.
When assembling, use a polysulfate bedding compound on both sides of the insulating material and some extra around where the bolts go. Torque the bolts all the way, don't wait a day to retighten them.
Use a spring washer only under the bolt heads, no flat washer which defeat the function of the spring washer. Use Tef-gel on the bolt threads.
Getting the holes in the exact position is tougher than it seems... follow all the advise given earlier in the thread for that because drill bits indeed walk sideways in aluminium.
I am not so positive that 1/4" drill bit is okay but never tapped that thick aluminium in SAE sizes. I would surely try to get the correct drill bit. The tables used for selecting the drill bit already come up with a smaller size bit because of the soft material. If you go smaller again from .257" to .25" then that might just become a problem while tapping an inch into the material (I don't know, but wouldn't be surprised either)