hi bluesman. A dropped D
slide is the best choice if thats the tuning you have chosen. Heavy glass or heavy brass would be my choice. I custom ordered a leaded glass dropped D slide that works very well for accustic and electric. Glass has a smooth. bright sound. I am waiting for Kirk to go back to Oz so he can start making his brass dropped D slides again. Brass has a darker sound and would fit better with the bluesguys you want to play. It should be happening in May. The heavier the slide, the more sustain you have and the easier it is to keep the slide on the strings. Read some of the posts here about slides for other options.
I think Diamond glass slides for dropped D go into mass production in May also. There is a link to their site here somewhere. Till then you could allways take a full length brass slide and cut it down to dropped D size. By the way, Kirk is working on a dropped D slide "how to" paid lessons course that will show you all aspects of playing in dropped D.
allthumbs