Shine Bright!
I bought a Varia almost a year ago, but I wasn’t a fan of the elastic straps used to secure the bulky mount to the seat post. So, I designed a custom 3D-printed holder that fits perfectly with the original Garmin interface — or the Mr. Control Garmin adapter I had lying around in my spare parts bin.
The design is tailored to fit the back of the 10mm offset BMC Teammachine SLR seatpost. The lower bolt tightens against the saddle clamp hardware, ensuring a secure and stable attachment.
I printed the part using PLA on my trusty Creality Ender 3. The hardware shown is temporary, as I plan to upgrade to sleek stainless steel bolts and nuts, replacing the standard ones pictured here.
The final version assembled on the seatpost and its design in the background. 
3D assembly in OneShape 
Sub-assembly showing the Garmin Varia interface fitted into the 3D printed part 
Mr Control interface version 
Garmi Varia interface version
The OnShape document is here: https://cad.onshape.com/documents/41f742d35fa21c714c959bbe/w/7f583f7ca0d614f82062f985/e/afc66c373dd7cecbcffaf570?configuration=List_it9oCS0RFO6aM6%3DGarmin_interface&renderMode=0&rightPanel=configPanel&uiState=6742f0c923fc5934d5c89fc4
Thingiverse:
https://www.thingiverse.com/thing:6844047