Cant you get yourself shortened a bit?
I think you need a GOOD joiner. The part that need to be raised is going to need the tie down raising, first. Thats pretty simple. Longer bolts, two bits of box or oblong section steel. Then the ramp part would need making is 18mm marine ply, with a decent understructure strong enough not to flex, cut accurately to fit over the tie down + a carpet thickness. The edges next to the door also need to allow a carpet to fold around it and down to the ground to make it fit right and look right. It needs to be only where you drive, with a slope abot 9 inches long to the rear, starting exactly behing the rear wheels when you are in the correct position.
I suggest making a cardboard template and include the ramp end and sides too. First!!! Sit it on the tie down for now. Then once its exacty right, cut it out to fit OVER the tie down, leaving carpet room. This will locate it and keep it correctly positioned when finished.
Then remove, take to upholsterer or a good carpet fitter, and get them to match the existing carpet or floor covering, or use that rubber stuff with the dimples... Should look factory once done. Like it was designed that way. It may need a ramp to the side edge too, towards the passenger seat sise so that you cat turn and manoever in the van. Will be a fiddly job, and you will see the ramp part as you open the driver side centre door. So that edge must be carpeted and flush with existing floor edge. See for shape,
download/file.php?id=8240&mode=view The ramp part would start directly behind the tiedown. And be about 10 inches long. And the same, 10 inches toward the side where the passenger seat is, directly after the tyre edge.
Failing that you will need to do the whole floor with a shallow ramp from the entry ramp into the van about 20 inches long.
Of course the simple solution would be a chair with lift, or a footplate that is electrically adjustable for angle AND length. Lift it and knees up a few inches to drive and or back behind the castors. This does all 3.