  We numerically examine run-and-tumble active matter particles in Casimir
geometries composed of two finite parallel walls. We find that there is an
attractive force between the two walls of a magnitude that increases with
increasing run length. The attraction exhibits an unusual exponential
dependence on the wall separation, and it arises due to a depletion of swimmers
in the region between the walls by a combination of the motion of the particles
along the walls and a geometric shadowing effect. This attraction is robust as
long as the wall separation is comparable to or smaller than the swimmer run
length, and is only slightly reduced by the inclusion of steric interactions
between swimmers. We also examine other geometries and find regimes in which
there is a crossover from attraction to repulsion between the walls as a
function of wall separation and run length.
