Large transitions (and other parts) can be easily and cheaply made out of EPS foam blocks by cutting them with a hot wire (running time 15m).
The rocket this transition was made for is my RS-2U.
Some useful resources:
- Search "hot wire foam cutter".
- Check out this do it yourself hot-wire setup.
- Hot Wire Foam Factory is where I bought my foam and glue.
- ACP Sales is where I bought my power supply.