Now that I'm armed with the knowledge PCBway has directly and indirectly given me, I thought I'd try to see if the university technicians could attempt to manufacture.

For the stock material, there's only really round bar on ebay (the place I've been finding the material stock in previous logs charges £9 for delivery). 35mm and 40mm sizes are suitable.

Next, now that the bore is 5mm, I have access to this internal grooving tool that has a rounded edge:

This should help with keeping a consistent internal pressure.

The only issue is that the max reach of the tool is 10mm and the middle internal groves are 10.6mm. Thus, I need to reduce the channels to 1mm and the height/spacing of the internal grooves to 1.2mm.