I was going to make some homemade mount for the roof tabs with steel channel and happen to have some 80/20 30 series t-slot from another project. So i cut a short piece and it slide into the factory tab without any play. So i drill a hole on one side 80/20 and a 1/4" carriage bolt fit perfectly into the slot. I drill another hole on the other side of the 80/20 and slotted it so i can slide in the second carriage bolt. If i would do it again, i would cut the 80/20 a bit longer maybe with center to center of the carriage bolt about 2-2.5". i barely have enough adjustment to match up to superstrut slotted holes with 1.5-1.75" CtC.
I then measure the height of the factory tab with 80/20 on it and determine about 25mm(1") spacer is good. But you can made the spacer as short as 21mm. I can't find any pvc or plastic square stock locally so i just use some oak 1-1/2" square from HD and cut to 25mm long. I either need to prime and paint those spacer for weatherproofing or buy some plastic square stock online in the future. I thought about using short piece of superstrut as the spacer but it will make the rack a bit tall than i want. the rest of the rack are made of two 10' 1-5/8 superstrut, eight 5' 13/16 superstrut(cut from four 10' strut), 3/8 hex bolt, washer, locking washer and superstrut 3/8 nuts. About $140 with majority of the cost going to buying the superstruts. It could be made cheaper if you can get used strut on CL.
Very clever! Looks like a pretty bombproof rack, and quite a bit cheaper to build than most. You would spend more than that on just the Vantech feet.
Some UHMW plastic should work nicely, or like you said - keep the oak if you prep and paint it well. What are you planning to mount that requires 8 crossbars?