Mavic only do the EN821 in 26 or 29 at present, but they have a budget line of UST rims in 27.5 – the XM419, EN421, and EN423. Quite a bit heavier than the high end offerings, and use a specific rim tape for each model, but do have real UST rim profiles with bead hooks. Last 2 digits in each case are internal rim width.
If I were you I’d look at DT EX471 XM401 rims – I built some EX471 rims up on old hubs recently and they are way stiffer than the rims they replaced, or the EN821s I have on another bike. Again they use tape (2 rounds of Gorilla Tape worked perfectly) but I prefer tape to the fiddly and heavy grommet approach Mavis use on their high end rims.
EX471 are 25mm internally and the XM401 are21mm internal.
