Like this ?
something along these lines.
I built myself a plenum this summer, I tried to mimic stock air box as much as possible (including velocity stacks which were also DIY). I'm still learning how to weld (I was on like 2nd or 3rd cylinder of argon).
I'm thinking about about chopping a front part and installing a water-air cooler or fabricating another one. See attached pics.
$1200-1300 for ebay intercooled plenums is too high. Sometimes I think about fabricating similar plenums and selling them for much lower price.
I spent less than $100 for aluminum (3mm sheet and 51mm piping) and like $10 for 5356 rods. I used like 1/3rd of sheet metal. It took about 2-3 days to fabricate the plenum. Intercooler core and end-tanks would be like $150 max.
So it's like $200 for aluminum and 16hours to fabricate a plenum. Selling it for $1200 means you get $1000 for 16hrs or $62 per hour.