Lots of different skid plate designs. What kind of wheeling will you be doing?
Myself, I'll be doing all but the most extreme rock crawling (dusy ersham trail, rubicon, etc, yes... johnson valley, CA... no) so I'm looking at 1/4" plate to cover my transmission and x-case. From experience, it seems the engine oil pan and surrounding areas rarely get hit so 1/8" plate reinforced w/ cross bracing ought to be strong enough.
Don't forget to re-enforce the mount points whether you use the OE mounts or build your own mounts.
Also, either use countersunk bolts in countersunk holes or i do recall seeing solid steel countersunk washers. Unfortunately I don't recall who offers them but I'm sure you can find them via a search.