Hey Jon, When building forms from that LVL material, you'll have much better luck keeping it straight using full 1.75" thickness rather than sanding it down to 1.5" . the last one i built i just used a router on each side along the edge in a few spots to get it to 1.5" to center the lams.
Edge banding the cut out is good too, as well as sealing it well. i use a Deft brush on lacquer, or Shellac. I buy it by the gallon pretty cheap and use it a lot for sealing all kinds of things.... Food for thought. Kirk