Green’s Dictionary of Slang

any temporary structure, esp. a stage, boxing ring etc, which can be assembled, then knocked down for assembly at another venue.

1864		P. Paterson Glimpses Real Life xxxiv 333: The theatre was what is called ‘a fit-up’, erected in the large room at [...] a small hotel of the town.
1889		Music Hall & Theatre Rev. 9 Mar. 60/2: I am catering principally for fancy dress balls and amateurs. I supply these latter with a complete fit-up, including costumes, wigs, scenery, limelight, &c.
1900		Sporting Times 10 Feb. 2/2: Must dress well on and off [stage], be handy with the brush, keep from drink, help with the fit-up.
1972		B. Rodgers Queens’ Vernacular 81: fit-up (Brit gay sl) 1. starring in one-night performances in different theaters 2. a one-night stand.
1977		G.F. Newman Villain’s Tale 36: There were managers and would-be promoters standing around the fit-up ring.