What is the grammar form with “にした” in this sentence?

Question

I have another sentence from 合格できる、日本語能力試験N２:

そこで私はいつも考えるのだが、各国にある日本大使館は日本人旅行者のため、こういう留学生、研究家を講師にした講座を現地で開いてくれないだろうか。

The part in the sentence where it says 「講師にした講座」 is hard for me to understand. I wondered why it cannot be 「講師にして講座」. Is there some grammar form in this sentence that explains why it should be 「講師にした講座」?

Answer 1

• Traditional grammar says (stupidly) that it is the 連体形 of the 助動詞 た.
• Some modern linguists say it is the past attributive form (連体形) of する used in a relative clause.
• I believe it is the past morpheme (attached to the stem of する) used in a relative clause.

Answer 2

Actually it can be. Think about this simplified sentence:

大使館は、研究家を講師にした講座を開く。

And, compare it to this one:

大使館は、研究家を講師にして講座を開く。

Both have almost the same meaning. However, in the former, the clause 研究家を講師にした modifies the noun 講座, while in the latter, the clause 研究家を講師にして modifies the verb 開く (or the clause 大使館は開く).