You’ve already had great advice on buying.
I want to recommend against building. My SO built my trunk and I LOVE it, but I wanted something for the barn only that had some extras (like door panels on the front, etc.). My barn also isn’t high end (most people just use Rubbermaid containers) so it could look a little homemade.
The end result was a big headache for SO, nearly 6 months of waiting when his job got in the way, and a box with some “character” that you get from something homemade. It also cost a lot. After wood, hardware, stain/paint, etc. we were probably in it for about what the used trunk will cost.
Again, I LOVE mine, and I would do it again, but not if I was at a barn where I wanted to blend it with real trunks.