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I have quickly thrown together this example, thanks to Knighty for the maths and code.
The attached files are for V1.21 custom formulas.

rename the two .txt files to .c files, and place in the custom_ocl_formulas folder file,  the .fract is the default setting file

A restriction with V1.21 custom formulas,  is that there are only 15 parameter boxes.

In this example 12 are used as follows:

0, 1, 2 are  pre-menger additions to z.x, z.,y, z.z
3, 4, 5 are  post-menger additions to z.x, z.,y, z.z
6, 7, 8 are  muliplications to a vector applied to z.x, z.,y, z.z in prism shape
12 is the menger sponge scale
13 is scaling a condition in the prism shape
14 turns on the prism shape by entering "1".

you can also use the three rotation parameters on the mandelbox tab.