I had the need to cut a lot of small diameter (1mm) heatshrink tube in 10mm sections so I designed this little jig.

It uses a standard metal razor/scraper blade (like these https://s.click.aliexpress.com/e/_c3Lgo3Of), one M3 8mm long screw and M3 nut, 2x 20mm long M4 hex head bolts and corresponding nuts and 2x ~12mm compression springs. The length of the bolts and springs can be longer to accommodate bigger diameter heatshrink tube.

Insert the M4 bolts and use a section of heatshrink tubing on the leaving only the top threads exposed. Insert the two springs. Mount the blade in between the two blade holder parts and use the M3 screw an nut to hold the parts together, insert this assembly over the bolts, insert the knob and secure with the two M3 nuts.