Great to see the 1st part out
To get more carbon into the joiner mould try drying the crap out of the carbon with a roller and paper towl before laying up.
Weigh the carbon before and after you wet it so you can get just the right amount of resin to carbon ,around 30% i think it is.
As for cutting it once it drys ,buy a cheap drop saw ,very quick and totally square cut .You will find lots of other uses for it .
keep up the good work.