If you do some research online, you will get some ideas there of various tack boxes that are already out there. How big are you planning to make it? Make sure you do a lot of research on the wood you decide to use.
When I build mine, I use a thinner plywood with more layers which made it about half the weight it would have been had I used a thicker wood. I also have wheels build on the back of it with handles so I can move my tack box myself without problems. The dimensions are also important. My trailer doesnt have a tack room so the sizes of my tack box is design to fit in the empty standing stall which converts that side into my tack area.