I have built a few of the tube traps using the instructions at
http://www.teresaudio.com/heaven/traps/traps.html.
They weren't that hard to build, but they weren't that easy either.
The problem for me, being a perfectionist, was the circle tops and bottoms, I used a router and a circle jig and cutting down 1/8 in at a time made a nice template, then I used a 'tracer' bit, mounted to a router in a table, and stacked the template on a roughly cut piece to make the others. It was time consuming and the circle gig, and template bit ended up costing me about $50 USD
I can settle the density question - I weighed a piece after removing the foil, and caluclated how many cubic feet were in the piece I weighed - (these were Knauff 1000deg pipe wrap) - it came out to 3Lb/cubic foot.
I made 2 approx 22 inch diameter, 2 inch wall thickness traps, and 4 appox 13 inch diameter 1.5" thickness traps.
I have to say I didn't notice a huge difference with them.
I will take some room measurements and post them at some point in the next few weeks.
In the mean time, I think the idea insulation rolls from the home store is a great one - as pointed out - it's cheap, easy, and easily reversible.
I'll also comment that The traps look ridiculous. I honestly think the marginal improvement in acoustics doesn't justify the look on my wifes face. There has got to be a better way....