We started with a fairly large box, cut oval holes for the head and arms, as well as a larger hole in the bottom with plenty of room to walk. We taped the edges of the box where we'd cut out the holes. In this case we used masking tape -- if I did this again I'd use something that would stay stuck better once painted, as the masking tape started to lift with the wet paint.
Next we took it outside and spray painted the box red (to make a red LEGO piece!). I used a Rustoleum spray paint ("Ultra Cover") in a bright red colour... it turned out perfectly and took about 3 light coats to really cover the box.
Now to make it really look like a big LEGO brick... we taped red SOLO cups that we had cut down to about half the height to the front of the box using red electrical tape. The bigger the box, the more 'bumps' you could use. We chose 6 bumps for this size box.
Finally, we printed a few LEGO logos that we found online, and glued those to the side of the 'brick'. We also taped one to a red baseball hat and dressed The Boy in a red shirt and black pants.
I think it turned out really well... certainly no mistaking what he was dressed up as, and he just LOVED the costume! It was worn many times after Halloween too...