As a trike owner I find that the mounting of accessories is always a challenge. Mounting a map case
presented a special problem.
My solution was to strap it around my waist. The strap is one of those 1-inch-wide woven types
(adjustable with a quick release). They're available at REI, Campmor, etc. as backpack tie-downs and
run about 4 ft. for about $2. I also spied them at HomeDepot after the fact.
The map case itself was an old kayak map case. Water-proof with a clip on each corner. The original
size accomodated an 11x14 inch sheet which I found to be too large for cycling so I simply cut the
case in half. The cut edge became the new opening, kept closed by adhesive-backed velcro strips.
Threaded the belt through the two remaining clips and voila!
The solution worked out well (for once).