How about a lighter smaller box

Say a 2 row Castagnari Lilly

Or a smaller 1 row on the size of a Melodeon/Max
but with lighter materials and perhaps fewer reeds

Also there is a simple trick to make a "suspension" strap for the left hand

Also have you considered going with 2 straps

Left thigh is what I prefer as well. Seems to be the standard method for Irish players etc.

Many players spread a small blanket across their lap as well.