I once observed the unfavorable consequences of using a similar homemade device made of plywood.   The device greatly changed the aerodynamics in the hood, causing formation of vortices/tubulence (because of the high flow rates) instead of the intended front to rear nearly laminar flow.  The investigators were using dispersible radioactive material in the hood.   The turbulence caused the microparticulates to be deposited on their sleeves.  Furthermore, each time the users withdrew their arms, a bolus of swirling air escaped the enclosure.  The result was (a) considerable contamination of lab coat sleeves, (b) deposition of radioactive material on the floor in front of the the hood, and (c) shoe contamination/tracking of the radioactive particulates.  After decontamination, smoke tests by professional industrial hygienists (who were not previously consulted as per policy)_demonstrated the (a) vortices/turbulence in the hood, and (b) the escape of smoke from the hood when the users even partially withdrew their arms.