The trial fitting was good and tight -- good enough for skirmishing. Could not, of course, let it go at that. Am Vityaz, after all, so I had to go for something much more -- the best fit possible.
First task was to build up the inner walls of the ABS handguards. This would give additional structural strength as well as a heftier, more realistic weight to the handguards.

Next was a secondary layer of steel-filled epoxy to provide a darker base color for painting, smoother planes for final finishing, and a harder surface that can stand more-than-ordinary abuse.

After some curing, the steel-filled epoxy coat was dremeled to the proper shape and readied for the second trial fit-up.

Here is a close up of the receiver tang without the spring slot.

And here is how the handguards looked after some rough shaping with the dremel.

Looking good, if I may say so myself.
I then took the handguards down again to cut a slot into the built up rear side walls of the lower handguard.

The slots were quite easily done with a bit of handsaw maneuvering and careful use of carving and craft knives.
Voila ! The handguards with the retention spring it its slot.

Here is the final fit up with the retention spring in the lower handguard.