It's not easy or fun, but you can do it yourself with an angle grinder and belt sander. My process is to get measurements of manufactured barrels from Colerain and Rice, and spend some time calculating how deep to cut to get the swamp I want and still maintain a safe thickness. I cut to the proper depth with a hack saw on all eight facets at multiple points along the barrel. Then I pull out the angle grinder and grind down nearly to the bottom of the cuts, and finish with the belt sander and draw filing. Go slow and easy, and don't be afraid to step away from it for a while.
It's definitely worth the money to just buy a swamped barrel, but sometimes you don't have the option.