We are just going to stick with the american standard thickness because 20mm would have to be milled down, which is custom, which cost more.

Generally, we just want to stick with sizes:
1/4" (6.35 millimeter)
1/2" (12.7 millimeter)
1" (25.4 millimeter)

There's a possibility of welding 1 (one) 1/2" plate and 1 (one) 1/4" plate together, which will achieve 19.05mm. Its very close to 20mm.