Just 2m and 70cm for now or do you want to support HF?
Do you have any idea what potential mounts you have available, such as flat areas, hand rails, etc?
Cheapest, easiest solution if you can is a mag mount dual band stuck on a A/C or swamp cooler or a metal end table on a deck. Next idea would be a clamp-on mount, maybe to a railing or fence.
If you want to do HF I'd probably do a screwdriver. In your case with the 857/897 I'd give the Yaesu ATAS-120 a shot. I have one of those that I used on my truck, they work fine and being an SO-239 type mount you can rig up something easily.
"Silent enim leges inter arma." -- Cicero