Try a fish trap with some bait. You can buy one or make one. How big a trap you need depends on the size of the pleco.
Basically, you need to make a tube and cap off each end with a cone-shaped funnel (the narrow part of the funnel should be inside the tube). Each funnel should have an opening just large enough to let the fish in. Once it's in, the pleco will have a heck of a hard time getting out. Whatever you make the trap out of be sure you drill lots of .25-.5 inch holes in it. This will allow for good water exchange and will allow the scent of the bait to permeate into the water.
For bait, just use a bunch of algae wafers.
For materials, you may want to consider chicken wire for the "tube" and maybe the top half of two large, cone-shaped plastic jugs for the cones? I'm sure you can find lots of products that come in those types of bottles at any store. Just make sure you clean them well.
Or, if you're careful in the way you cut the wire and can grind down any sharp edges, you could make the two cones out of chicken wire as well.