A.H.
My MIL did this one year for my neices. She put it together herself by getting a trunk at Hobby Lobby or Michael's. Then she bought up a lot of costumes after Halloween when they were really marked down. The kids loved it!
We found a ready made Dora the Explorer trunk at Walmart last year as a gift for my kids' friend. It was about $15 but I haven't seen it since.
Good luck!
A.