Even if this question has already an accepted answer,
let me link this interesting Wikipedia page for everybody’s enjoyment.
The problem that in principle Esperanto doesn’t have one-letter words can be easily overcome applying the rules of standard Pilish: for instance, the sequence
1 2 can be encoded by a 12-letter word.
In my following attempt, however, I resorted to another trick.
a finaĵ’ adjektiva,
tiele sen manko funkcias
fleksebla, belsona Esperanto.
Translation: “Here it is: o (is a) noun, a an adjective ending, as makes verbs: flexible, nice-sounding Esperanto works in this way without shortcomings.”
Explanation: The trick I resorted to is simple: I used the basic endings o, a and as as independent words. This was even envisioned by Zamenhof himself, in a famous answer: endings and suffixes are independent words (here, p.48) As a final insider joke, I used the adjectives flexible and nice-sounding, which are part of a well-known, outdated slogan from several decades ago: simpla, fleksebla, belsona, vere internacia en siaj elementoj... and so on.