"We make it."
would the other version of déanaimid é be déanann muid é, or can't you do that with this verb?
I still don't understand why 'sí' or 'sé' aren't correct, but I guessed right this time. If anyone has some clarification for me I would be grateful.
sé/sí specify who is doing the "making" when used after a verb. é/í are used to refer to objects being made (unless the copula, "is", is used). You can tell that é is appropriate here because the verb, déanaimid, has already inherently specified the "maker" as we/us. sé/sí can mean it(masc.)/it(fem.) but only when used appropriately. Example: Déanann sé cáca (It made cake).
Tangent: The general trend I have noticed is that when verbs involve me, as in the "I" and "we/us" cases, there is a specific conjugation where the "mé" or "muid/sinn" is contracted into the verb. Example: Táim, Ithim, Siúlaimid, Rithimid. For everything else, namely tú/sé/sí/sibh/siad (you(sg.)/he/she/you(pl.)/them), there is a general conjugation followed by the appropriate pronoun. Example: Tá tú/sé/sí/sibh/siad , Itheann tú/sé/sí/sibh/siad, etc...