J.F,

Ive a set of the BVM air blue retracts. They are less than 30mm in height. If you pm me i can measure them up if you think they may be of interest. You'd need to check with BVM if they would take the projected weight, but theyre well made and were used in their 17lb Takeoff weight Electra.
( Im in billingshurst so fairly local if you wanted to try them in situ before ordering a set for yourself.)