"improvised" would be the formal synonym for "makeshift", I think :)

"Improvised" is the word that popped into my mind. I agree with Mikkel that "makeshift" is not informal; and I just checked a dictionary and it does not flag it as informal. "Makeshift" is a compound of two Old English words, while "improvised" is derived from Latin, so as usual the Latin-derived word is a bit more formal. There's a slight difference in meaning. "Makeshift" suggests that something doesn't work very well and probably won't work for very long. "Improvised" means only that a creative solution was found, using materials at hand, that weren't intended for the purpose. An "improvised" solution might be very good.

"Makeshift" means temporary or provisional. "Permanent," "solid," and "substantial" are all antonyms.

I just want to add that I don't think there is anything informal about "makeshift". It's neutral in terms of formality in my opinion.

temporary