Well I grinded the last 15 series levels I needed for the coffer yesterday, just so I can nag a bit lmao. And yes, the best advice that was posted few pages back about learning to enjoy/not care about losing is the most effective advice if you are looking to grind the thing in a short amount of time.

I think they should ultimately let you buy the coffer, or individual pieces of the glam, with trophy crystals. This would mean that if you just happen to accumulate enough trophy crystals during several series but never manage to hit 25 for whatever reason, you can still obtain the glam. You get 19k trophy crystals in the series rewards before level 25, so I think 20k trophy crystals for the glam coffer would be just fine (so max amount of trophy cystals you can hold at one time), and still a considerable grind that would force you to partake in PvP nonetheless. Alternatively they could sell each individual piece for 5k or so trophy crystals, so if you are only interested in one or two pieces, you don't have to do the whole grind just for a few pieces, but instead go for 5-6 levels instead.

Considering there are also several other options to spend your trophy crystals for, I don't think this would be a completely bad alternative.