Why not make collet blocks for 3MT collets?

[Toolmaker51]

I like the ER32 collet blocks. A 5C or any similar shaped collet will only collapse .005" under nominal. ER collets, except for the smallest sizes will collapse 1 mm. A set of ER collets has a continuous range of holding diameters from 1mm under the smallest to the largest.

Exactly, plus a fairly easy female contour to make. Generally speaking, double split collets have better range and grip, compared to single split holders. It's all in the ability to collapse; doubles contract in a progressive cylindrical manner. ER's for example; adept holding work or tooling.
Singles are only truely cylindrical at one size. Deviating from that causes collapsed or sprung holders.