Lawsuits for Treestands are rampant (Walmart, Cabelas etc require a 20,000,000 limit per occurrence from manufacturers) and the insurance is quiet expensive. If it were me I would build the box and explain to them that they will have to install the base themselves. The accidents usually occur from the height exposure and collapse around the base. The insurance for tree stand manufacturing...especially from wood...is going to be high.
A waiver doesn't help much for a deceased father or a paralyzed person who will require 24 hour care until death.
I am aware of the liability issue. I would only build blinds for ground use and I would include in writing that putting the stands on legs would make it unsafe. But as stated above you can sue anyone for anything. There are a lot of lawyers out there that give the lawyer business a very bad name. I use to have a lot of respect for attornies but only a few have my respect now, Perry Mason and Mattlock.