Angle gauge, steel rule, saw, file, knife. Apply tools to trunking in the manner prescribed by the art and craft, and a perfect joint results. There's no magic shortcut - electrical installation work is a craft and it requires skill and practice. A joiner can make joints in wood of arbitrary angles that fit together exactly and look smart; an electrician should be able to do the same with electrical materials. Unfotunately, the very kind of work that uses PVC mini trunking is not the kind to draw the attention of the most skilled craftsmen, so one doesn't see precision work in mini around the place.
Wind back the clock 100 years and you would see electricians making complex joints in wooden casing with flyovers, double mitres etc. That wasn't something you could learn from a book, and was nigh impossible for the DIYer. When I look back at my first few years' steel trunking work it's got some rough edges (not literally!). It has taken 20 years to get good at it and now I don't do much on the tools and avoid first fix like the plague, I'm losing it again for want of practice.
If it's any consolation, when my mentor asked me to tube up an area in 20mm BE steel con while he was in hospital, with funny angles in roof trusses, he made sure there was just enough tube. I made so many misbends that I secretly bought another bundle of conduit and cut up and took away most of the errors!
