There are cones for ukuleles. Haven't tried them, but did a quick Google search:
Did a build with a 6" lowe cone and its fantastic.
FYI. I have a list of resonator cone makers/sellers on this CBN discussion group: Home Made Resonator Boxes 101. If you know of any more, let me know and I can add to the list. The site also has some other links of interest as well.
Good luck getting cones & cover plates from Backporchmojo. The proprietor has recently closed down his business due to health problems.
Mark Lillo said:
If somebody is interested, i propose to build a soldered metal resonator. I love tin soldering, and can made a photo tutorial step by step. For me is a way to "thank you everybody" for all that i have discovered here. Of course a very advanced cone resonator, Helmoltz based.