Got started on the tonneau, since I don't have any time to lose here. (It took two 4x8's of OSB, at $10/piece)
I've got four boards screwed to the underside to prevent it from sliding more than 10-15mm. Later added a couple 1-1/2" holes on the rear edge to help me slide it on/off, although it is extremely unwieldy.
I have been wondering how to secure it down. I have two stake pockets at the front top corners of the bed, and two die-down hooks at the rear lower corners. I think I will drill a few more 1-1/2" holes, and then a few in a line horizontally between them. Then I can throw a good, thick bungee cord through each hole, hook it on the stake pocket/tie-down, pull it taut and stretch it to the furthest possible hole. This should pull the (already heavy) tonneau to the bed enough to keep it from moving or making noise.