Check out the links Eranrund posted a few comments
down in this thread I recently created about exact roof rack mounting point locations/dimensions. I have largely copied his build, but for a 148 HR long (not extended) and with inside corner gussets on the cross rails instead of a second inner row of 1515 and 4 hole corner plates like he used to deal with an interference issue between the DIYVan mounting brackets and the REC Alpha panels we both independently concluded were the best option. He just beat me to it!
I probably should have made my own brackets and used aluminum square tubing for the frame or my initial idea of just using powder coated steel unistruts from Home Depot. I would have saved a bundle, but I'm exhausted from the endless build and the DIYVan + 1515-LS rack solution will go much faster, be much lighter/stronger, and is visually appealing. I think I'm about $980 in cost so far on the rack and mounting (another $1.2k on the Alphas). I have everything I need except a bunch of stainless carriage bolts and I already have a ton of nylocs, so probably ~$1k for the whole rack getup. For EL, I'd estimate $1.2K. I bet I could have made a comparably strong rack, a bit heavier with unistrut and custom brackets for $350, and a lot of time and effort.
But this lets me get back on the road sooner, and 1515 makes it very easy to add things like pull rings at all four corners of the rack so I can easily carbineer on a tarp shade canopy to the front, back, or either side of the van. Plus custom lighting if I decided I need it (see Eran's setup), or whatever else I want to attach to the sides and front of the 1515.