i did my interior stripped the paint, primed, hand painted a good thick layer of paint and primer so no water/rust
i bought my ready made carpet, cut left over carpet and used that stuff for insulation on the floor.
the wheel walls, box section and side curtains i just used 'Selleys glue' in a tin, heat/water resistant, directly on readymade carpet
mini is kept in house garage, no plans on ripping it out soon
original layers on walls:
metal
prime
paint
glue
hessian (that shitty dusty brown potato sack stuff)
glue
rubber mat
original layers on floor/box section:
metal
prime
paint
glue
rubber mat
this is what i noticed when ripping everything out. check my site maybe it will give you a little idea on what im talking about
http://www.angelfire.com/mech/ducho/
Anton's Just_Another_Mini site has my work progress its not working now ill email him about it
http://www.justanothermini.tk/
velcro wont be useful on floor - too weak, gets tired
velcro strips on box section, wheel walls should work. it comes in a roll like duct tape and you stick it on both sides. im not sure about sticking velcro on carpet, maybe you could handstitch it onto the carpet