Securing plywood over carpet

[PJYAY]

Recently brought XTR B9 berlingo  and in the months leading up to and following purchase have been reading and watching everything I can for inspiration on making a removable camper conversion.

Plan is to remove the rear seats and replace with a box with hinged plywood sheets that will unfold to bridge the footwell between the box and the front seats which will be completely folded down. 
Took the plunge and brought some ply and fittings  and have found  that after cutting slots and holes through 12mm ply for the seat fittings I can have it extend under the rear seats yet still allow them to lock and unlock into place (well the left and right seat any way, can't see where the middle is catching but will have another play tomorrow as ran out of daylight today).
Plan is to fit some Tee-nuts to the underside of the sheet and can then easily bolt and unbolt and furniture as required keeping it nice and secure.

Reason for posting is I'm just wondering if anyone has any tricks for securing the plywood to the floor ideally keeping the carpet in place. Current best guess is removing lashing points and getting longer bolts of same diameter and bolting the wood at those points, biggest issue I can see will be trying to accurately drill a hole in the wood that lines up with these points.

Anyone got other suggestions/see other issues with my plan/tips to accurately drill those holes?

[Sol]

Get a replacement rear floor liner, in vinyl or carpet, about £35 and then use that as a template. It has the lashing holes already cut out, so you could use those positions after laying it down and work out how far in, the bolt is in the lashing hole. Also could then (after marking the lashing circles in the wood from the second liner/carpet), take one lashing out, lay it in the hole with the bolt missing, draw exactly where the bolt hole is on the plate of the lashing ring, bingo. Make it a bit bigger when you drill, to give some wiggle room for all the bolts in the board, and it should drop right in.

[jemselectrical]

Recently brought XTR B9 berlingo  and in the months leading up to and following purchase have been reading and watching everything I can for inspiration on making a removable camper conversion.

Plan is to remove the rear seats and replace with a box with hinged plywood sheets that will unfold to bridge the footwell between the box and the front seats which will be completely folded down. 
Took the plunge and brought some ply and fittings  and have found  that after cutting slots and holes through 12mm ply for the seat fittings I can have it extend under the rear seats yet still allow them to lock and unlock into place (well the left and right seat any way, can't see where the middle is catching but will have another play tomorrow as ran out of daylight today).
Plan is to fit some Tee-nuts to the underside of the sheet and can then easily bolt and unbolt and furniture as required keeping it nice and secure.

Reason for posting is I'm just wondering if anyone has any tricks for securing the plywood to the floor ideally keeping the carpet in place. Current best guess is removing lashing points and getting longer bolts of same diameter and bolting the wood at those points, biggest issue I can see will be trying to accurately drill a hole in the wood that lines up with these points.

Anyone got other suggestions/see other issues with my plan/tips to accurately drill those holes?

I used 40mm long M8 stainless countersunk bolts to fix the ply down using the tie-down bolt points. Won't move anywhere if you use these. You do need to shim the back face of the ply with extra blocks of ply, to take up the gap, otherwise it'll distort the floor ply and not tighten down        

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[jemselectrical]

Here's the back face of the ply with the blocks fitted:    

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[PJYAY]

Thanks to both of you, some really helpful tips.
Might try making a paper template first but will look at second hand liners as a back up plan if it all goes Pete Tong.