I was in at Bunnings today looking at their plastic pot offerings in hope of finding a basic 9" to 6" ducting transition when I stumbled across these types of pots.
This one was not wide enough at the mouth for my transition but it would make a reasonable BMH.
Pros
Good basic shape, made of strong plastic - I think its HDPE.
Unlike many pots the holes are only on the bottom, Most pots have gaps up the sides as well (whereby they would leak air)
The base suits a 6"/150mm duct (I'll show you how further down) and the opening has a nice wide radiused edge.
Cons
They are longer than the optimised 6"/150 mm BMH so will take up more space - but that might be OK for a lathe or Belt sander situation and maybe even for some cabinet ports
They are wider than the optimised 6"/150 mm BMH so would not project higher air intake speeds as far as the optimised hood.
The radius of curvature is not quite right
But Hey - for $6 they have to be better than a naked piece of duct.
Attachment 410667
Here's how I would convert these.
These photos are of the 6 = 9" transition I am making has the same size 6" base as the above pot, but this one has an opening that is a little wider than the above pot
Cut the bottom out with tin snips or a strong pair of scissors. Leave a couple of mm of lip for a longer/tighter contact with the 6"/150 mm ducting.
These pots have 4 little ribs on the inside corner and these need to be completely cut away with a utility knife.
Put the pot on top of a piece of PVC ducting and while rotating the pot, heat up the outside edge of the pot base with a hot air gum until the plastic develops a sheen.
Don't let it get too hot or it will melt and ruin the edge.
To get a feel fr how hot practice on the scrap you cut out from the base.
Attachment 410668
Then, push and stretch the edge of the pot base over the PVC duct - considerable force is required.
push the pot right down to the end of the pipe.
You could radius the inside edge of the duct for an improved flow and can seal any small gaps with something like silicone.
When it cools it grips the PVC ducting like a shrink fit tube.
It took me about 5 minutes to make.
For my transition the 9" (250 mm OD) duct slides neatly up to the first ridge line inside this pot so it should be OK as a reducer.
Attachment 410671