Mactac from the dollar store works fine. You can just use brown paper bag type stuff which you can buy on a roll. Sometimes you can get newspaper end rolls and then just wrap it around several times to cover and use the clear packing tape to secure all edges and seams. Putting tape on all the edges will help protect them and they won't wear out as quickly.

If you look online for places that sell boxes such as www.uline.ca you might find you can get some extra thick but plain boxes. Then all you would have to do is seal them closed. In the long run that might be the cheapest way to go compared to buying the brick blocks. Also by the time you buy the mactac you will have spent a few dollars too.