After some thought, I've an alternative approach to mount the red aerial filter on my C5, it's simply a thin metal ring like the following:-
The 3 red circles are 3 holes to be connected to the black metal cell via 3 screws, the black metal cell has 4mm width. The green rectangles are to be cut from the blue ring, which matches the two protruding lock from the objective end of the C5.
3 yellow holes are to be drilled and threaded to connect with the 3 red holes above via 3 screws. The black ring has 4mm width.
The red filter is recessed from the top of the cell for around 6mm which is more than enough to hold the filter on the objective of the C5.