Green’s Dictionary of Slang

1. to assemble or put together in a haphazard, bodged manner.

1834		D. Crockett Narrative of Life of D.C. (1934) 104: I got the boatsmen all to go out with me to where I was going to settle, and we slap’d up a cabin in little or no time.
1912		Bulletin (Sydney) 12 Sept. 52/1: We’re expecting people in the afternoon. [...] I slap up scones for the sacrificial feast, and, in between basting the spluttering leg, try to make our one living room look less sordid.
1921		F. Grose A Rough Y.M. Bloke 26: Strike me! Yer should er seen the feed they slapped up that night! It was a beaut.

2. see slap n.² (2)