The debate continues on-line about the purpose and merits of the spring platform arrangment, and even about the number of supporting bolts to use. The springs are to allow easy levelling, and possibly absorbe accidental impact of the extruder head. The argument in favour of three bolts instead of four is that you can more easily level the platfurm with three spring loaded bolts vs four. I'd agree with this but would thing the down-side is a larger section of the square platform is now unsupported and may flex depending on the rigidity of your platform material.
I'm not sure of the need to have any leveling capability at all though. If your rods are alligned on the horizontal plane, and parallel to each other, your Z axis accurately perpendicular to the horizontal, and your X axis perpendicular to Y, then why are you levelling the platform? Surely you should get the Y axis right and step the 'print platform' up to the desired height with fixed height rigid risers from the Y-axis bushings.
For me, the four bolts with springs design will do to get started, but I've beefed up the bolts and springs. I'm choosing a 'mushroom head square neck' bolt which secure nicely and have a low-profile. I could have gotten away with 5mm bolts but had some 6mm to hand.
Here's my construction method including the attachment of the bushings... Time will tell how well it works!
The upper and lower boards were clamped together and four holes drilled. The positioning of these holes isn't actually critical. Just mark the boards so you can assemble in the same orientation as you drilled. The round holes were then squared out with a needle file to receive the bolt neck.
To position the bushings accurately I put two scrap lengths of wood together and drilled two 8mm holes in them at the desired distance apart for the Y rods. This is an easy way to get the holes perfectly alligned. I then temporarily fit the bushings and rods as per the photo.
I used Araldite 2 part epoxy to secure the bushings to the under-side of the base platform, allowing it to set over night.
Finally, I fitted the springs, attached the top plate to the base plate and secured it with some nylock-nuts.