Mark Dresser started out with basically a P-Bass pickup installed on his bass. Mark did eventually get pickups from Bill Bartolini after Fred Firth showed him the pickups he made him. A sculptor named Don Jacobson made him a pickup that I guess attached to the bass and he used that pickup for a while. He's now using pickups built by Kent Mclagan that are installed in his fingerboard called the Giffus.
I've had moderate success getting bitones to project in some recordings where I have a mic placed up near my fingerboard. I've had some success doing two handed continuous glasses and tapping like techniques in performances with some drummers. Otherwise I only try to use them when I'm not playing with a drummer. For me I know it's something I have to be very sensitive about when to use appropriately since I don't have and should never have a Giffus of my one. Everything Mark Dresser and Bert Turetzky have done though is absolutely inspiring and always worth exploring.
Installing a pickup in or under the nut (alla Fred Firth) could be a great place to start! Otherwise I think it's going to be hard to find a fairly cheap way to amplify them. It's either going to involve some sort of pickup installed in the fingerboard and or nut, or an attachable pickup. There's a great interview Mark did for Contrabass Conversations (I'll attach the link) last November and he mentions that he still is exploring bitones as of now. I hope I'm not discouraging you though! I do love bitones and use them in some contexts but it's something as of now I'm not focused too heavily on. Try contacting Mark though to talk about some ideas. He's a wonderful person and, I find, very open to sharing some of his knowledge. Best of luck in your explorations!
So I hope I don't get too wordy with my explanation of a bitone. The definition itself is just what you would expect, two tones being sounded at once. Mark Dresser is the one for bitones. Bert Turetzky talks about them in his book "The Contemporary Contrabass" and I guess used them (@damonsmith do you know of any pieces/recordings of Turetzky where he uses bitones?) Also check out Bert Turetzky. He's Mark Dresser's teacher. Here is a link to a video from Mark Dresser's instructional video "Guts" describing some of the techniques he uses to play bitones.