A suggestion: Using the acrylic bending table discussed in the other thread you won’t be able to make a round tube. But you could make a polygon with 8~ 12 angles approximating a circle and get a single edge to line up on the flat of the first surface, although the last bend may be less acute so the edge lay flatter/more perpendicular to each other - a better option would be to put in a rabbit on the edge so when the vacuum is pulled the “glued” joint is under less strain. The polygon would allow for easier clamping and alignment of surfaces for “gluing” (I assume you will use a solvent to melt the joints together). The end caps could be square so long as they cover the ends. The flats of the square end would also provide a stable base so it would roll.
A semi-circle on a flat piece would have to edge joints whereas a polygon would have one, cutting in half the glued surface area, excluding the end pieces which would be two in both cases.
Have you tried the vacuum pump in automotive used for evacuating A/C systems, you be able to get down to the pressure you need (not sure if it is a 1,2, or three 3 pump.)