Crooked Stic, I would disagree on how critical the measurements are, but would want to emphasize at how important a glass..or if need be phenolic I beam is for strength.
I have never had one fail (I have done a lot of sleeve TD's!) so I can't really say how small the roughed in parts could be, but I have made them quite small.
I have also used many different woods to do these risers. If properly re-enforced, I would say most any wood could work for a riser.
I'm not a big fan of any of the other two piece systems or I would be using those instead.
Price isn't an important factor if your product is poor. But it is nice to know that the materials for the carbon/glass sleave system is less than half of any other system out there.
BIgjim