Here's a method for a permanently rigged deck with low C of G.
......The setup that my buddies do is to cut pieces of (marine) ply (5/8~3/4) to loosely fit the void space ahead and another behind the frame (front and back). They 'round' the edges of the boards with a 'router' + curved bit. Holes are also bored near the edges to be adjacent to any D rings on the inside of the tubes to suspend the board by straps ....... the height of the boards adjusted by the strap lengths and keep it clear of the floor.
They bore the holes with one of those 'stepped' drill bits (~ 1" + Dia - makes good 'clean' holes) then use the curved router bit to create nice smooth edges to the holes (strap friendly). Various other holes can be added for gear tie-down points here and there.
(A suitable stepped drill bit and router bits are fairly inexpensive at Harbor Freight and a cheap used router from there or Pawn shop)
If you want to get fancy and/or not go the marine ply route (tad spendy), finish the boards with 2pac fiberglass resin - applied with a paint roller (half width roller is most manageable). Very durable! Resin can be found at pretty much any auto parts store. This finish seals the boards very well, makes them very durable, waterproof, splinter free and the roller leaves a 'popply' non slip finish.
Note on the resin .......... don't go wild on the hardener - it'll harden waaaaaay quicker than you think, wear gloves, do in well ventilated area! Full width rollers can be cut in half - regular latex rollers are fine. Be prepared to get the roller off the holder when the resin starts to harden - you'll use several B4 you're finished - don't mix too much resin at one time.
I have a Aire Cougar cat which has quad tubes, 2 each side. I bought a 4x10' 3/4 ply panel and made 2' wide full length side decks and added many holes using the same methods mentioned above.
I added a whole bunch of holes 'hither and thither' for freight strapping and also tube lashing (the D rings are not positioned very well for the frame tie-down!)
Don't have pics at this time, sorry.