The part seen in the photo above, from the OP, is from a Leitz Focomat V35 enlarger isn't it? I'm not going to dismantle mine now to check, but I think I recall that the dichroic filters are after the heat-filter in the light path and not in the hottest part of the head.

However, removing one of the filters being used for the variable-contrast filtration will mean that you need under-lens filters to use VC paper anyway -- so wouldn't that be the easiest place to put the red filter? (Plus you get one in the under-lens set anyway, so don't have to make anything except a simple bracket to support the filter-holder).