My middle row seats are about 53cm when folded flat (only 2 of mine fold flat)

I found this image on Amazon of another solution 51cm in height.

Assume if you added 18mm ply on top of that it'd be near enough level with the folder seats
Is 18mm ply overkill?

 

Seems like a bit of a faff to be honest but it looks simple enough to make out of simple steel square section and yes its very expensive as you say, have you considered the Stitches and steel bed it pulls up in to the roof when not in use, you just need to have a simple frame to support it at the back your front seats will do the rest

Drop-Down Carbon Bunk

The Stitches + Steel Carbon Drop Down Bed is the ultimate, less-is-more campervan solution for anyone looking to retain the use of the functional space within the back of the van, while moments later a means of sleeping accommodation. Convert your van for camping. ➕ FOR ADVENTURES ON THE ROAD ➕

stitchesandsteel.com

 

You've probably seen this one?

VW Transporter T5 | T5.1 | T6 | T6.1 Kombi bed system | CapsNkeys

Tried and tested, these Volkswagen T5 | T5.1 | T6 | T6.1 Kombi beds are built to last, powder coated frames, easy installation, versatile and fully removable. Free UK mainland shipping, excludes highlands and some other postcodes. For these areas please ask for a separate shipping cost. We...

www.capsnkeys.co.uk

Shame these guys don't ship outside UK, would have been ideal for £380 plus your own boards.

Maybe send to a mate in UK and arrange your own courier to Eire. Probably getting pricey that though.

Maybe a useful image as a breakdown idea anyway.

 

It uses the bolt holes for the tie downs that are already in the van
So you make up 2 frames for the sides & then some brace bars to go across between the 2
Main benefit is that it can pack away pretty flat when out of the van

That is essentially what I am planning to do with some square tubing. I've free access to a welder too, so that helps. The plan was just as you've shown, to secure the frame with the tie down points (hence my thread a couple of months back about keeping or covering them with the flooring). Haven't decided between OSB or flooring chipboards for laying across the top, nor on the thickness.

I need to be able to remove the whole thing when not in use as it would get in the way for day to day usage.