You seem to have a Myford sized lathe, but not sure what toolpost.
I have something similar to this one from Chronos, but mine, rather than a wedge, holds the blade with an angled flat plate, but the same broad principal
https://www.chronos.ltd.uk/product/spare-parting-blade-holder-myford-quick-change-toolpost/

Note that it's intended for the slightly wedge shaped HSS parting blades, so holds my hacksaw blade at a slight angle from the vertical, but since I'm only using it for shallow grooves, it's never been a problem. I leave the blade in the holder when re-sharpening.

If you were to make one from scratch, just milling the slot with a dovetail cutter on the bottom edge, would leave a vertical face for the blade to butt against.

Bill

I made one for doing some deep fins on a cylinder head. I don't have any dimensions to hand but this is what it looked like. The blade was clamped by small cap head screws, the holder then held in a Dickson Quick Change Tool Holder as normal

It's quite a while back but I think I hand ground the end of a slot drill to give the slight angle (dovetail) on both lips which worked well enough to give a good grip on the blade.

Hope that's of help

Tug

Here's my home made holder. Made from 1" x 1/8" and 1" x 3/16" BMS held together by 4 x 4 BA csk. skt. head screws.

The blade sits across the 2 lower screws. I use it a lot for small stuff including steel.

It will easily part 1/2" dia. brass.Sharpened in seconds of course.

Must admit that for the purpose of undercuts that the OP wants to do I've taken to using a DCMT or DCGT insert to do both the initial turning and then the undercut which saves having to change tools and also gives a nice rounded fillet to the root of the undercut which should be a lot stronger than a square cut one.

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Having worked on Swiss instruments I noticed that the did not do undercuts on shafts, but did recess facing galleries. These better to retain any lubricant' To my mind an undercut gives a weak point on the shaft at the junction of main body!

Not if you only undercut to the root diameter and have a radius on the left corner of the tool (looking from the top).

Clive and Geoff's 4 screw clamp type for old hacksaw blades. I am certain that type appeared in one of the older traditional books on model engineering but unfortunately I cannot remember which one. It could be as far back as a "Duplex" book. Not tried making one but I think the design shows promise of effective Ness. Probably won't bother making one unless a special need arises as I'm very happy with my RDG Tools version. The HSS blade has been sharpened both ends in opposite directions so it could be switched, with it's holder, from front to rear toolposts without repositioning the blade. Looking forward to completing machining the rear toolpost to try.

John

A point to bear in mind – I have found this by trying it – a plain saw blade with the teeth ground off is likely to jam not very far into a deep cut because it has no side clearance. Even more so if not absolutely perpendicular both vertically and horizontally to the axis.

It may best to leave the teeth on, and arrange them at the top, so they shave the walls of the kerf a little (just as they did when they were part of a saw.) Obviously creating a top rake, needed only at the cutting edge, will remove the first one or perhaps two, teeth but that should not be a problem.

Arranging the blade teeth-down is of course very likely to create a swarf-trap, also jamming the tool.