volume substract causes gaps, unable to mesh

Ron

I imported a model from .dxf (by means of .igs and .sat) to Gambit, with about 10 differently shaped solids. The flow domain is supposed to be a rectangular cube with these solids in it, as blockings to the flow. My way of solving it was creating a big rectangle and do a boolean substract of those blocks from the rectangle. However, this action results in many short gaps and strange shaped surfaces (for example "P" shaped, where the bottom part of the "p" has width of 10E-3 or less)inside the cube, and therefore continuous failure of meshing. Even if after the solid healing and making tolerant, the volume still refuses to mesh. I wonder is there any way to solve this problem, either some tips on meshing, or on getting around from the boolean substract while still keeping the effect of those solid blockings and walls to the flow?

Thanks,