I used 1x4 lumber for my current layout. It is more than sufficient to climb on top of. It is not L girder but just typical framing. Mine are two feet on center with 1/2 inch plywood on top. I predrilled all my wire holes on a drill press before assembly. I however put everything together with kreg joints aka pocket holes. I am absolutely impressed with the strength. I did not glue anything so I can modify without too much fuss.
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