I think a Lansky with 'custom' bent rods would be a good way of setting a sharp initial edge in the absence of a belt grinder.. While it's been about a year since my last Busse kin purchase, up to then the edges came concrete chopping ready and sliced like bananas.. Thinning them out helps no end.

The sharpest edge I ever got was using the mouse-mat but it was always hit or (mostly) miss for me. It seems the secret is keeping the spine low and just using the knife's weight not pushing down at all. Perhaps thick leather would be better- but I've not tried it.. (Less liable to round off the edge as the blade can't sink in so much) My DF is sporting a Lansky V edge right now and it seems fine. My much abused Tramontina machete also has a V edge and that is fine too having had the brown stuff knocked out of it!

In the field I just take medium and fine ceramic sticks.. Maybe not razor but good enough for little effort!