I threw this together more with a view to touring than bikepacking if I'm honest. I used it on the BB winter event as a first trip as a bit of a test to see if it was workable and wouldn't collapse in a heap.

Mapboard by Richard Picton, on Flickr

It's an old chopping board with a bit of attention from my bandsaw. The depth is sized to fit an A4 printed map sheet folded over.

I wanted to fit my Garmin computer on as well as there wasn't room on the bars to fit it easily. It's an Alpkit stem top cap mount, with a Nyloc nut on the back to hold it in place.
Edit - One of these - https://www.alpkit.com/products/love-mud-trace

Garmin mount by Richard Picton, on Flickr

I figured the easiest way to fix it the bike with A. a bit of adjustment and B. be quick and easy to fit and remove between different bikes was again from Alpkit - one of the Mudlove stem cap bottle cage mounts.
This one - https://www.alpkit.com/products/love-mud-bheesty

Mapboard bracket by Richard Picton, on Flickr

The other hole is just so I can prod an Allen key through to bolt it to the top of the steerer. Simples.

like that idea of combined map/garmin mount... nicely done sir!

Ta!