Which paper are you using for testing? I know some of them (Slavich, for example) specifically state that only red light is safe. For those I use a DIY 635nm red LED safelight.

Your description sounds thorough and correct to me. Is your tape lifting up from the heat perhaps? Is it truly opaque?

I suppose you could try temporarily covering the outside of the outer filters with some taped down red Rubylith. What you need to find is some level of baseline configuration that works so you can then work backwards from that to isolate the problem.