I've put a bit of thought into the pieces I'll need to build a suitable frame out of aluminium extrusion.

It'll look cool, plus give me a nice, easy to assemble/adjust frame. It'll be nice to use the channels to run cables too. I've decided to target 15mm x 15mm extrusion as it's small enough to hold comfortably without being too flimsy.

Dimensions based on 15mm material

Slide Section calculations
400mm - slide space (360mm travel [60mm between positions], +40mm for tolerance? Can be adjusted with another inserted piece?
+15mm - for the 'water key' brace
+15mm - for handle
+50mm - for mouthpiece area

Top Section calculations
200mm - to form the shoulder mount.
+15mm - for top bar
+15mm - for brace bar

Parts to order
480mm bar x 2
230mm bar x 2
100mm bar x 4

All in all it'll give me an instrument length of  ~ 74cm once the two half pieces are joined together - nice and small, but enough space to mount everything and get a decent slide area.

Now i just need to find a cheap supplier of 15x15 extrusion :)