https://makerworld.com/en/models/1142374#profileId-1144909
Some time ago I built a 3D printable drag knife tool. The key differentiator is that it uses an insert to allow different blades to be used. It seems to work really well and is very rigid. I have added it to my Makerworld account and it’s free to download. The build and print instructions are also there. I use this in my Shapeoko pro with an 8mm collet because I have 8mm carbon fibre rod on hand for the spindle (and the 608 bearings are a push fit 8m interior diameter for the spindle) but you could print a spindle at any diameter very easily.
The insert idea is so that you can print an insert for a scalpel blade, or a twin blade, or a rotating cutter, or a cardboard folder roller, or a sharpie, or, and this is a major thing I am hoping to solve, a reciprocating blade driven by a rechargeable battery.
Anyhow if it is useful to you then great. If you have a blade you would want a model of an insert building then ping me a dimensioned sketch of the blade and I’ll build an insert.
ps. Make sure you understand how a drag knife works. The router should always be disconnected from power supply when using a drag knife.