At the left is a "Clement" focuser. This one uses a Gortite bellows to move the eyepiece.  It has the lowest 1" low profile of any focuser on the market. The focusing knob is parallel to the eyepiece axis, and it takes 30 turns of the knob to give a full 3" of travel.   The focusing is extremely smooth without imageshift and it does not need a lock to hold it in focus. In fact once focused there is no slippage. The Turcite-X leadscrew nut is the only moving or sliding part.
No part of this focuser protrudes into the telescope. My only concern is that I don't know how well the gortite bellows will perform in -40° temperatures.
Put your mouse over here to extend the bellows half way, over here to extend them all the way, and over here to reset the bellows back to their no extension position.