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.
'91 Pickup - Imelda
'08 Tacoma TRD - Donna
'09 Kawasaki KLR650
'12 Gunnar Rockhound 29
"Our task must be to free ourselves from this prison by widening our circle of compassion to embrace all living creatures and the whole of nature in its beauty. Nobody is able to achieve this completely, but the striving for such achievement is in itself a part of the liberation and a foundation for inner security. " --Albert Einstein