These layered plinths are also quite time consuming and labour intensive to make, especially for a woodwork clutz like me. :-(   While I think it's worth the effort, I would not reject a quicker solution.  :-)

Lenco decks up to about 74 were mounted on 4 springs, so the deck plate was sprung from the plinth.  This technique was almost universal across the popular idler drive decks, including Garrard, BSR who generally used just 3 springs.  I remember using this type of deck with the transit screws wound up to defeat the bouncy springs.  It improved the 'footfall' performance, much like Thorens putting foam inside the springs on TD150's to stiffen the suspension.   It also couples the deck to the environment making feedback more of a problem.

The use of springs on the deck plate continued into the early belt drives from Pioneer, Sansui etc., the popular Dual 505 range also has rubber damped springs. 

Nowadays belt drive decks are either sprung chassis separate from the motor, or hard mounted and high mass.  Some makers have developed low vibration motors that are fixed to the sprung chassis and these decks are reckoned to be better in respect of speed stability, because the speed anomalies from a fixed motor, floating chassis design are avoided.

Garrard 301s today are often mounted on a medium mass board that is then decoupled from the rest of the plinth using rubber blocks, this is a different approach to the layered high mass solid plinths mounted on cones/feet. 

R.Bruil mounts his Technics SP10 on a medium mass board that is sprung from the plinth, a design he says was pioneered by Mitch Cotter.

Back to Lencos, they changed the L75/78 plinth around 74.  The deck was now screwed to the plinth and the plinth sprung from the base board on 4 springs. 

This had to involve extra cost so I presume it was more than a marketing ploy.  The earlier design had the deck plate completely undamped and free to resonate on the springs.  However the new design bolts the deck plate to a medium mass plinth (still a box) that is then isolated from the environment.  The hidden extra 'feature' is that instead of using the 4mm screws (the old spring hangers) to fix the deck down, they fitted wooden blocks inside the plinth, and fixed the deck by screws thro' the lower moulded area.  This will provide some damping to the deck plate.  To see if this design has any benefits, I bought a GL75 version on epay.  More here.

I think the L75 plinth using the blocks under the moulded panel to fix the deck is a definite improvement to fixing the deck with the 4mm spring hanger bolts.  In this design the upper plate is supported by 4 stiff rubber grommets.

Supporting the deck by the lower moulded area where the bearing and motor attach has got to be good.  In order to create a high mass plinth that uses this technique a new pattern matching the geography of the underside of the lower panel and controls is required. 

I spent an hour creating a paper pattern for the L70 trying to leave as much support for the metal plate around the bearing as possible.  After a patch or 2 this was the result.

 From the paper pattern I cut out this chipboard template.

Using the template I marked out and cut 3 layers of chipboard from an old table top.  Daisy was not impressed!

The deck sits on the top board with the recessed panel in direct contact with the top layer.

View underneath the top board, as much contact area as the mechanism allowed.  The contact area could maybe be improved with some tweaking of the paper pattern, but this will have to do for now.

Now for the major problem of this design - the top panel of the deck is about 11mm off the top board.  A further board is required to take up this gap and provide good support for the top panel.

I used a 10mm MDF board and some thick card shims to provide support for the top panel.  When the screws are fixed to the lower moulded area they pull the top panel tight onto the mdf board.  The 4mm bolts can also be fitted if I need to, I am leaving them out for now. 

Time for a glue fest! ............... don't forget your mittens ................. it's cold outside .........