I've decided to use spline roadbed and further decided to use Homasote as the spline material for the following reasons:
- Price is similar to Masonite for a given length of spline
- Easier assembly and cleaner using drywall screws instead of glue and many clamps
- Drywall screw assembly means easier disassembly / adjustment if needed
- Potentially quieter
- Can drive track nails / spikes into the material easily

One difficulty with any spline approach is the cutting. 4X8 material is difficult to manage on a table saw meaning inconsistently straight cuts - which are required. It's also dangerous.

The contraption here allows consistently straight cuts using a circular saw. The material is slid under the guide block until it hits the material stop. In this photo, it is set up to cut 2" strips (with my saw). It has multiple mounting positions to accomodate cutting "dimensional" 1x3 and 1x4's from plywood.