Perhaps start with what you wish to drive them with, your room and how much LF extension/sensitivity you want out of them.
RL horns are more lively than a bass reflex for example. This means they're not extremely neutral but they can have high sensitivity and a lively character. They'll never be as well behaved as a multi-way front loaded horn for example, but the simplicity of the concept appeals to some.
I have a Fostex FE206En for which I built an extension for a an existing horn that tunes to 90Hz. Horns drop off fairly rapidly below their cut off frequency so a little subwoofer assistance is warranted (and the same would apply with this design). On the upshot they are ~105dB sensitive in the mid-bass and up which makes for a combo that is extremely efficient and punchy.
As always, Hoffmann's Iron law applies: efficiency, low bass extension, small enclosure: choose two.