DIY air filtration unit for <$200

As payback to the lasering community, I'm sharing my "plans" (such as they are) for everyone to copy. Well, not so much "plans" as pictures of the completed unit, but it should be obvious how to recreate something similar.

Due to poor planning on my part and a lack of any real sleep, I made one or two errors in the construction of this unit, but I'm happy to say it works like a dream. After a solid hour and a half of engraving a maple block at 100% power, there was only the faintest aroma of wood in the air, and not the tiniest touch of smoke haze. It all took a few weekends, each with a few hours here and there to allow for drying of the caulk, and the cost was under $200

Materials:
1) Trashcan ($25)
2) 4" PVC piping, 2' ($6)
3) 3.5" PVC cap ($4)
4) 3/4" PVC piping, 2' ($2)
5) HEPA filter ($23)
6) Furnace filter ($10)
7) Silicone caulk ($3)
8) PVC cleaner/cement ($8)
9) Activated charcoal, 50 lbs. ($115)

Tools:
1) Caulk gun
2) Saw or Dremel

The great thing about this project is there is no real measuring to be done, just align things by eye or what "feels" right. The first pic is of the outside of the unit. It's nothing more than a typical plastic kitchen trashcan. Two 4" PVC tubes allow air in/out, with a ring of the 4" pipe chopped into short segments glued both inside and outside of the trashhcan to help act as a barrier to air (with the caulk) as well as give the joint more strength.

The second pic shows an inside shot so you can see a few 3/4" supports, roughly 6" in height, simply taped to the walls. These supports, along with friction from an over-size cut, hold up the furnace filter, just above the 4" inlet pipe.

This furnace filter, shown in the third pic, is sold as a cut-to-size sheet of a basic blue filter and a charcoal filter. The final filter sandwich consists of a plastic support mesh, blue scrubber, black charcoal, and another support mesh, all tied together with the included plastic ties. I cut it about 1/2" too large all around to give me a friction fit.

The final pic shows the whole shebang put together, minus the activated charcoal, with the HEPA filter installed. The HEPA filter was purchased at Sears as a replacement filter for one of their shop vacs, and the end was plugged using the PVC cap attached to the inside of a short length of the 4" piping.

Air from the machine comes into the bottom of the machine, the blower connects to the top. Use about a 3" layer of activated charcoal on top of the furnace filter... the edge supports and the friction fit will hold up that small amount without issue. Dirty, smoke-filled air comes in at the bottom, the furnace filter traps large particle, like scrap paper, etc., the charcoal above it traps the smoke particles and/or noxious plastic fumes, and the HEPA filter traps anything missed.

If possible, weigh the charcoal charge and the whole assembly as a separate item. Weigh the entire piece, charcoal and all, from time to time... when the weight of the charcoal roughly doubles, it's time to change it out.

I hope I didn't miss anything, but if I did I'm sure someone will ask...