I used curtain blackout material on the prototype, but that fabric was very heavy and more prone to damage than the nylon.
Believe me, the cost of the grow tents make them very favorable by comparison, if they are indeed lightproof. I have yet to find the definitive answer to that question.
Other wetplate photographers have used the grow tents as portable darkrooms, but not out in direct sunlight. But, they may work well under suitable conditions of subdued light interiors.