OhioMike here,
The box I made was 6"W X 12"L X 2"Deep, with a hole drilled on one end to plug the hose into. For the lid I took 2 pieces of aluminum and just set them on the top. That way I could control the width of the slit by moving them up and down seperately. When I first started using it I taped the lids to the box to get a good seal. As jigmeister said, I no longer tape them down, a shop vac gives you all the vacuum need
without sealing the lid, alittle too much & it will pull the plastic right thru.
I also use a foot pedal to controll the on-off