This uses the existing space of a SS. Works for the 505 and on.
Materials
1 Sheet 2x4 melamine ($15)
4 Screw inserts 5/16" (<$1)
4 bolts 5/15" thread (or whatever size you use for the inserts) approx 1" long
4 lock washers
Front and rear
floating table rails ($65 plus sh) If you want to use the main table rails for more stability, that bumps the price to about $103
Epoxy
Note--You could just get the rails for a 505 for $24 and attach but it would mean extra work drilling and measuring (not as easy as bolting on the 520 rails)
Also, your choice of router table insert (Kreg, Incra, Etc.) ($60 to $100) and fasteners levelers ($25)
And---
Two SS tables plus connecting tubes. I describe using the main and a floating,
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Cut the shelf in to two pieces, 24 by table depth (approx 21 15/16"??)
Stack and epoxy together.
After cured, cut out the hole for your router plate. (Template with router , jigsaw or whatever!)
If just using the 520 rails,....
on a level surface turn the main table and a floating table upside down with space in between for your router table,
You want the top of the router table aligned with the other tables so why not let gravity replace measuring
Thread tubes through each main table rail
Slide on the "extra" set of rails with holes down and facing each other (since they will attach the router table)
Slide the floating table on to the tubes to make a router table sandwich.
On the sides of the router table, move the extra rails to somewhere near midway (yes I know mine are shown closer to the main table but I got them close using SketchUp when I had the router plate closer to the main table and left them when I moved the insert to the center of the table - in the real world I'd place them closer to directly across from the router for support).
mark the location of the rail fastener holes. See, no measuring!
Disassemble
Locate and drill the holes and insert the thread inserts.
Use appropriate length and thread bolts I may use thread lock to keep the bolts from working loose.
Voila, you have a router table that can use the SS tables for extra space and support and it all fits with in the foot print of the SS. Because you used the rails, it can fit anywhere along the SS.
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Double tilt, separate router table, buy SS's table, convert a main table (as did Steve), make your own extension.
Lots of choices at lots of price points!