Mark,
Firstly, welcome to ribnet!
Two things - Firstly yes, a trunk will empy the boat a LOT quickwer than that wee hole. Secondly, is that bung in your photo attatched to the inside of the hull, or the cavilty under the floor? It looks a little too low to be a hull drain.......
As for the trunks, there are plenty of people will sell you a ready to fit trunk. (I got a pair of them form Humber to replace my knackered ones for (I think) about £40. No idea what your internal layout of the boat is, but leading the release string forward to the console is also a useful mod....
In the meantime, I suggest you pack a bucket. (seriously - I swamped my old SR4 "sans trunks", and was sooooo glad I had one with me!)