If I understand correctly, you want to put your backflow preventer below grade. That is a no can do unless something has changed in the industry that I'm not familiar with because it needs to be at minimum 12" above the highest point of irrigation discharge on the property. In the City & County of Denver it is illegal to have it at or below grade & imagine that's the case for most municipalities. What you've seen are indeed valve boxes for valves but if you've seen a commercial backflow in one I'd be really surprised. There are industrial enclosures that are steel tube & perforated sheet, powder coated forest green that stand between 24" & 36" & those cover freestanding backflows for larger commerical properties & parks. If you'd like I can send a pm with my contact information,