My question is on topic because it is about tightness:

How do you assemble the spline and track with the
material? I've got a tear sheet with the instructions
but still can't fathom the tightness part.

Don't say "just like doing a screen" because I've never
done that either!

Meantime I got a cheap piece of hd fiberboard and am
cutting to shape to do the glue up way.