Pleached Photinia Red Robin

Hi, I am posting here on the off chance someone may be able to help! I have a row of pleached photinia red robin which have grown nicely since they were planted in 2015. Unfortunately one of trees has died (it was looking unwell last year and all the leaves fell off in autumn leaving a sad sight) so I removed it a couple of weeks ago. I am trying to track down another the same size with a clear stem of 1.65m trained onto a 1.2 x 1.2m frame which seems to be an awkward size. I have looked at online nurseries and most seem either full standard (with 1.8 - 2m clear stem or 3/4 sized pleached which have a stem of 1.5m) I was wondering if anyone knows of or runs a nursery with plants that may meet my (annoyingly difficult!) requirement? Could a slightly taller tree be planted a little deeper? I'm assuming not as the trunk will rot?! I don't want to end up with one odd tree at the end of the row! Any advise gratefully received. Thanks, Alex