I designed and built the one shown in the Front Sight article, the article featured our club. Here in The Netherlands we mostly shoot indoors, having portable bullet traps gives you an enormous amount of extra options to build nice stages on a small indoor range.
Most of the portable traps in use here are completely made out of steel, some even with venetian blind style traps in them. Solid as a rock, but also really heavy. I personally don't have any welding or metal work skills what so ever, so I came up with the current design.
Simplest way to describe the design is that it's a box made out of 2x3 beams holding a sandwich of underlayment, a 1/4" thick Hardox 500 (AR 500 grade) steel, two 1" thick granular rubber tiles and another sheet of underlayment on the front. A small L frame around the steel plate saves the splash from the bullets disintegrating from eating up the wooden frame. Total cost of one of these traps is around 125 euro, of which most was for the Hardox steel plate. We have two of those in use for over two years already, they have been used once a week at our club training sessions and a handfull of matches so far. They are holding up great.
Some clips of how these were used in matches:
If you have any more questions let me know, I do intend to make a full DIY guide for these things, hope to be able to make some time for that soon.