What I do:
1). Any pedalboard or case with a wood bottom. I like plywood covered with black ozite (cabinet carpet).
2). Get some tin strapping from any hardware store, snip off short pieces just long enough to protrude from the edge of the box you are mounting.
note - Some guys use bike chain links, the problem with that is they're one fixed length... depending on the pedal you might need longer or shorter.
3). Undo bottom case screws from your pedal, insert bit of strapping under (6 on the FX-8 ). Put case screws back into pedal, holding the strapping tight in place.
4). Screw pedal tight down onto the board with shorty woodscrews.
I've used this method instead of Velcro for all kinds of pedals over the years. Easy, cheap, super strong and works on any size/weight of pedal, even big heavy ones like the FX8. Plus you don't end up with permanent velcro adhesive on the bottom when you pull it off your board in future.
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EDIT - what if you have a nice Anvil-style road case with a lift-off lid that's a perfect size for a pedalboard, but it doesn't have a plywood bottom?
* This was the case for my old pedalboard (as seen in my video below, right beside my FX8 pedalboard)
1). Cut out some plywood to fit inside. I'd use no thinner than 3/8", but probably don't need thicker than 1/2".
2). Wrap it in black Ozite (cab carpet). Looks pro and totally hides screw holes left over when you inevitably remove/reposition pedals in future. Use 3M spray adhesive for the surface, wrap the edges and corners same as when cab building.
Now, you could just place this board loose inside your case and be done with it, but I went one better...
3). Get some good size rubber feet like the ones used on the bottom of a cabinet.
4). Put your new plywood board inside the case, drill a hole in each corner right through the case AND the plywood board.
5). BOLT (not woodscrew) the rubber feet through the case onto the plywood, right through the case bottom. This locks the board to the case and adds nice non-slip rubber feet at the same time, so it won't slip around or scratch the floor. You could use woodscrews for this, but since it's the whole pedalboard I used t-nuts and bolts to make it super strong.
You can see both my pedalboard cases here, zip forward to around 2:20 or so...
