How thick is 355 skidplates?

Mine were probably 5/16-3/8 of an inch new. A little less now.

 

In metric that's 8-9.5

 

Yes. That was in mm. So 8.5 mm. That was in aluminum. It may be different in other materials.

 

I made mine out of HDPE. $15 for a 24"x24" sheet which allowed me to make 5 pair. You could uses 3/8" if desired.

Corrosion Resistant HDPE Sheets 24x24 by VALUE BRAND - Plastic Sheets, Film, and Flat Stock by Zoro Tools Industrial Supplies

 

The ones made by Hill Engineering are 1/4" thick in either aluminum or black HDPE plastic. They are beveled on all sides, use 4 nominal 1/4" SS countersunk machine bolts, nuts and lock washers drilled through the bumper and then through two supplied 10 gauge SS plates to engage each pair of bolts as a "washer".

 

Pretty much exactly as I made mine. You can pay$175 for the Hill units or make 5 pair for about $5 a set, including screw.
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I was just going to say my hill ones were quarter inch I think.

 

DO a search and see my post Skid Plate DYI Part 1 I posted in 2008.
Fairly easy if you have some basic carpentry tools

I used 1/4" HDPE