Question for the overlander types

I'm looking at the idea of improving the accessibility of my boot tray by having a second floor that slides it's own length out of the back and locks open. This would serve 2 purposes one holding the back door open and two making better access to my edc so it's less of a chore to take out every night and easier to get at the same when needed at work which is good for my back not having to lift things at full stretch. My idea was to just have a flat surface that moves and use the existing sides and bulkhead in their current fixed condition to maximise surface area. That would mean running slides flat under the tray which is beneficial for clearance of the cill without stealing too much headroom.  That's the design, this is the first issue I have come across, I can't find any slides that will take around 200kg to 250kg when used flat, they are good vertically but not horizontal without using 4 or 6 of them which then gets really expensive. Yes I could redesign and have vertical slides but it wastes space that I could otherwise have for cargo.

I have not found either the money nor the practicality for slides yet. But I have seen folks with slides a bit above the door sill so there is still access to the full depth of the rear boot. by raising the slide out, it can also improve on work height for cooking.
I bought cheap plastic boxes at first but now changed to these (or similar) that do not break, for the trailer:
www.plasticboxshop.co.uk/boxes-for-indus...hYPm05PQ2Eix_xNDeFK_
Later also bying for the rear of the jimny If I find ones that fit well enough.
I have changed for the rear seats that fold flat and made a floor behind it so there is a full flat surface to put the boxes on.

A friend of mine made a sliding drawer for an Amarok for the Red Cross. it covers the full back floor of the pickup bed and is big and heavy and full of gear but slides in and out beautiful.
He made Nylon sliding profiles from a milled out T-bar sliding in a U-bar and it is hands down the most amazing precise and well functioning thing.

My existing rear deck is set to slightly fall to the back door but it's still just under the cill by about 5mm just enough to stop small things rolling out on a gentle incline so using the dimensions of the slides I have found would allow them to clear the cill on their second stage without any other modifications. What did occur to me last night, could it be possible to use 2 shorter slides as inboard supports especially if they were able to extend in both directions but I don't know if they even make ones that do?