Summary

Folding Scalpel v3
Ok, so I went back to the drawing board, and studied folding knives to try to get a better working lock.
There are Blade Holders for #3 and #4 scalpel handles
You can get a good assortment of these blades for cheap here:

https://www.amazon.com/gp/product/B078Q2QRDX/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

and Here:

https://www.amazon.com/gp/product/B00OTKSUN6/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

I added a Rocker bar that has a pivot.

• fixed the Scalpel Holder to lock into the Rocker,
• adjusted the fit of the Blade to be tighter and not wiggle.
• made the screw holes a tiny bit smaller to take up slop.
• Also added a Tensioner to the Handle Bottom which turns out to just be decorative because the Rocker Bar flexes enough by itself with PETG.
  If you find that your material (PLA) doesn't flex enough, you can cut the Tensioner away from the Handle Bottom body to create a flex point. I used a Hacksaw blade. (see the picture)
  Wait to do this last if you even need to do it.
  It's necessary to use Plyers to grip the Blade while pushing it onto the Holder. You will need to push it on, flex the Tang down, then pull the blade out a little to get it to seat.
  Be careful!

I tried to enlarge all the features and still keep the knife small, but because they can be tiny, I suggest cleaning up slow and careful.
I had to take sand paper to the Rocker Bar Post to get it to fit.
I used These settings for Layer Width:
Default Extrusion Width = .45
First Layer width = .42
Perimeter width = .35
External Perimeter width = .4
Infill = .3
Solid Infill = .4
Top Solid Infil = .4
Infill/perimeter overlap = 40%
First Layer HEIGHT = .16
all Layer Heights after = .15
Vertical Walls = 6
Top and Bottom solid layers = 4 & 4
Infill = 5% Rectilinear
Kept speeds down to 50mm/s.
I did not use any X/Y compensation or Elephant foot compensation.
You might want to use a 3mm brim, but my bed is pretty sticky, so I didn't need to.
My Kobra isn't perfectly tuned, but it's decent, and these settings are how I get the parts to fit together.
If your having problems you might be able to adjust the X,Y compensation by .01 or -.01 incriments.
You will need 2 x M4 counter sunk screws at 12mm long
and 2x M4 nuts

https://www.amazon.com/gp/product/B083Q7ZR7D/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

This is probably still a work in progress, so If you feel like remixing it to make it better, please do. I'm no knife maker, just having fun!
Enjoy.