You might consider, instead of letters which are hearsay and inadmissible as evidence, to read and follow Illinois Supreme Court Rule 191 about affidavits. Affidavits can be made part of the file.
It requires that affidavits shall be “made from personal knowledge of the affiant; shall set forth with particularity the facts (not opinion or conclusions).
The parties may, if they wish, use supporting affidavits. Supreme Court Rule 191
sets out the requirement for evidentiary affidavits. This is what an attorney might do in the right circumstance.