It all depends on what you mean by 'outside surface finish is important' - MDF can get you shiny provided you want to do a little painting after. MDF will not have nearly the dimensional perfection of aluminum either - but it will totally work if you seal it well with epoxy and use a good release system over primer - or a surface film (like PTFE film) for release.
If you're trying to do this cheap and tolerance and strength aren't super critical, you could probably do fine with a bagged wet-layup in MDF molds - things would be tight doing the layup though! You could even make the molds by hand with a table-saw and some body filler. It all depends on how good you need it.
Infusion would be an option but 500mm is a bit far to infuse and it's more trouble. Infusing across the tube would be fine if you design-in a thicker split-line for the two outermost plies of carbon to turn out and act as resin flow media and vacuum outlet. This would be easier and less risky than end-to-end but there is a small cosmetic penalty. Everybody is suggesting pre-preg because for something this small it is much easier to control and keep tidy.
What type of laminate are you after? Would a 60% uni in the 0-direction and the rest +/-45 and one woven surface ply work?
Like: (from the outside)
200g at 0/90
300g uni at 0
300g uni at 0
200g at +/-45
300g uni at 0
200g at +/-45
300g uni at 0
200g at +/-45
This would be easy enough to wet-lay with the +/- 45's lapped - or laid out in one long staggered stack the full circumference of the tube and rolled around the internal bag. It might be messy with wet-layup but you could also use braided sleeve for the +/- plies.
Two pieces is tough - too bad you can't buy it somewhere. Hexagonal would be an off the shelf thing but I've never seen octagonal!
The problem is that once you go down the short-cut road you'll never get anything as nice as pre-preg in Steve's aluminum mold - but it'll take more time and probably cost almost the same money in the end. Wood CNCs can cut aluminum if you do it right - and machine shops could whip this out pretty fast. And low temp pre-preg can be cured in aluminum molds with one of those silicone heater strips like used for 3D printer beds stuck to one mold half.