Well.... there is this awesome 5 gallon by fishfanman , all full of beautiful coral and happily housing 2 clownfish. I wouldn't recommend this to just anybody, but knowing your love, respect and experience with aquaria, you may be able to add a 3rd fish. Just make sure to put that fish in before the clowns, and probably best to go with a bottom dweller.
Also, my 20 gallon reef is a closed system with a single
HOB, an AC70. If you're thinking of another HOB, why not just get a larger one and keep everything in it? That would be one less tube sticking into your tank anyhow. In the AC70 I have a large bag of
Matrix and a large ball of chaeto. There's still room too! I recently upset everyone by moving and mounting corals, so there's carbon in the HOB at the moment as well. And all this fits even with the little baffle we installed so a piece of floss padding can be in there, so mechanical filtration is possible too. Just don't forget, sponges and floss turn into
nitrate factories, so I use it when I need it. If long term use is necessary, floss and toss it is.