The shaded area in the drawing above shows the PVC blower I added. I used 3/4" pipe and fittings, plus one 1" coupler for the shop-vac input. I punched four 1-1/8" holes in the sander lid; the PVC elbows, caps and couplers hold the whole thing in place. I drilled 1/8" holes at 3/4" intervals along the blower pipes and pointed the rows of holes at about 45 degrees to shoot the air between the drum and pinch roller. Note that you must remove the "dust scoop" over the last pressure roller to make room for the rear blower pipe.
Don't use a really powerful shop-vac for this - you don't want the incoming air to overcome the vacuum of the dust collector.