I’ve spent quite a bit of time modding the UHK with keycaps. There are a number of my posts on reddit. (Back then there was no UHK forum 
)
I hope it’s ok if I link to some of those posts from here. You’ll find some tips there what to look for.
https://www.reddit.com/r/MechanicalKeyboards/comments/8rro7i/uhk_with_canvas_xda/
https://www.reddit.com/r/MechanicalKeyboards/comments/97hqt4/ultimate_nuclear_hacking/
https://www.reddit.com/r/MechanicalKeyboards/comments/9ecnk4/ultimate_godspeed_uhk_as_the_ultimate_flight/
https://www.reddit.com/r/MechanicalKeyboards/comments/9n7i9f/pulse_sa_and_honey_round_56_go_well_together_on/
https://www.reddit.com/r/MechanicalKeyboards/comments/li8lsr/feeling_ultimately_split_today/
https://www.reddit.com/r/MechanicalKeyboards/comments/9w6wes/i_felt_kind_of_blue_because_i_could_not_make_this/
https://www.reddit.com/r/MechanicalKeyboards/comments/m3jczn/finally_found_sa_convex_space_bars_in_175u/
https://www.reddit.com/r/MechanicalKeyboards/comments/eqmpy7/ultimate_hacking_oblivion/
Right now I am working on a “black-on-black” layout:
The right hand side of the UHK is difficult, particularly the top right corner backspace. Same-row profiles are easier (XDA, DSA). Backlight adds another twist of difficulty.
Feel free to shoot questions. Most of the time it is a lot of hunt and seek until you find the particular set that works, or often you need to combine a few.