[reuelt]

also the design in the second picture seems to have an issue as well. It seems that the acrylic screen/cover is used to run the sled against the fence. The piece to be routed is sticking out of the base and it has no back support (unless a sacrificial piece is used).
I am considering the below design in which the side of the sled situated next to the fence and ahead of the purple
clamp base (yellow block) is cut in such away that would not allow the bit to reach the base of the sled because there is nothing there
(that yellow block was cut out)

Is this router coping sled design OK ?
NO REALLY! Sorry.
In my new routing table which I am designing, there will be 2 parallel mitre slots (both with T-slots). So the runners will only have a cross-piece for hold down clamps. There will be no need of MDF nor Acrylic nor plywood or any sled of any thickness since the workpiece will be supported by the 2 mitre track runners which will be made the same level as the router table top itself.

(I bought 2 Aluminum Mitre Tracks to be precisely embedded in 2 precise parallel DADOS of correct depths to be routed out by my small CMC router.
My table top will be 38mm thick Free of Charge "sink cut-out" of a post-form kitchen worktop laminate).

 

[reuelt]

No really
So what is wrong with it...

If I am stuck with that router table, I would buy a ER20 collet extension so that I can place a stationary piece of wood of same thinkness as a thicker sled - with a round hole of almost zero clearance and put over the router bit. Safety and work. quality

(main reason why I want a new router table - read my comment #3 again)

 

[reuelt]

I looked at that bit extension and a good one could be quite expensive. Besides that you need two sizes 1/2 and 1/4

I paid about USD30 for a good one from UK bundled with 1/2", 1/4", 1/8" collets.
I also bought a cheap 13 pieces ER20 metric collet set 1mm to 13mm (made in China) for another USD20

With an ER20 collect extender, I could use CHEAP router bits (shank dimension precision becomes irrelevant) and I also need not remove the 2 column strings from my plunge router. Easy to change bits above the table. My sled could be nominal 3/4" plywood. (Actually, only 18mm now in Australia)

 

[reuelt]

Looks like it should work fine, Mine does. It’s only 1/4…

But nobody use 1/4" cross-cut sleds for their table saw. It's almost always 3/4" (now 18mm).

 

[reuelt]

Triton RT300 router table would be a better design. Sled and table are at the same level.

 

[reuelt]

Here is how much I can lift my router - do you think I can/should go with a nominal 1/4 or 5/8 or even 1/2 plywood for the base of the coping jig? Would I still need an extension bit?

Go for 1/2" plywood sled. Hard wood workpieces are NOT cheap.
Collet extension need not be bought immediately.
I bought mine from Axminster UK (Made in the UK)

Axcaliber Router Collet Extension (1/2" Shank)

The problem that most customers encounter when using a router inverted in a table set-up is that, even when fully plunged, the router collet is still some way below the surface of the table limiting the depth of cut. We decided that we would produce a...

www.axminstertools.com

It is cheaper than the "muscle chuck" and in the long run I saved a lot on router bits because ER20 collets DO NOT need router bits with precise shanks diameters available from only a very few expensive router bit makers. An 1/2" ER20 collet will perfectly mount a 12mm shank bit and a 1/4" ER20 collet will mount a 6mm bit perfectly too. You will never hear of ER20 collets getting stuck in the shaft and people asking for help in ROUTER FORUMS.