Since my last post, I thought some of you would be curious as to how well my home-made dojo has survived. In short, the dojo is performing very well, and seems unaffected by extremes in temperature (after all, this is a garage). If you notice from the photos, I decided to take down one post on the interior side of the mats, as the car door didn't agree with its placement. I did keep the exterior post, so that I could still utilize the "ukemi training pole", but more importantly, the exterior post is anchored to the wall, and the frame that surrounds the mats. The weight of the mats on the sub-floor would not result in much if any shifting, but I think the frame and anchoring was the right thing to do for such a small footprint; a larger set up would not likely require such additional work.
How does it feel?
In comparison to the "other dojo" where I train, my garage dojo is definitely taut (more BMW than Cadillac), but certainly not uncomfortable for hard falls. In fact, I used 33 blocks per 4'x8' area, so I expected a slightly harder surface as the foam performance would be very fast.
I had used a foam underlay between the sub-floor and the tatami mats, primarily to protect the mats from the deck screws. Another option, although more expensive, is carpet underlay, which can be used to tweak the hard-to-soft response of the mats. The "other dojo" I attend uses carpet underlay and there is a distinctive difference in response upon landing (I may try it out one day on my own mats).
In the end, I am quite happy with my dojo, and it serves me well. Each person is different, and I recommend trying a variety of materials or thicknesses under the tatami until you find the right mix.
My Dojo now: