Hey Ted! If you mean to ask whether you could sit up from one of the vertical sidebins and have enough headroom under the Leer 122, I think that would be a no. You’d have to build the sidebins to be a little shorter. At least with my truck, that would be the case. Perhaps with a full-size pickup you would have enough room thanks to the overall size increase of the canopy as well.
Use the scrap piece of plywood to map out the angle for the speaker stand legs. Decide the height you want for the base of the speakers, and make a mark on the plywood (32 in. for us). Mark the depth of the speaker (13 in.). Lay the 1x4 board assembly on the plywood, lining it up with both the corner of the plywood and the 13 in. mark from the edge. Scribe along the 1x4 board assembly, take the assembly away, and find your angle with a protractor or speed square (17 degrees for us). Cut that angle onto one end of the 1x4 board assembly.
For example, on your dual battery, the farm just purchased a brand new 2013 Chevy 3500HD with a dump bed. It has dual batteries. How was it installed? They grounded both batteries to the frame and connected the positives with a very large fuse. ~15k later its still working. While its not recommended to do it this way, I think it would work fine in your case. Add the marine battery somewhere in your shell and run a heavy positive from the original battery. Put a large fuse in it(I’d research to see the number, I can’t remember off the top of my head) and your isolator switch inline (probably 2nd battery side of the fuse).
The refrigeration setup in your van is an area where you really do get what you pay for, whether you’re paying in money or in time. The best options are the most expensive. The cheapest options are a pain and/or don’t work very well. And if you try to save money with a DIY refrigerator setup, you could end up spending a lot of time on installation.