Hi Cowboy
The only thing that would hold your brain strom back would be the round router base plate is not over size that's to say it's flush with the Alum. base of the router the norm ,it needs to be over size by a min. of 1/2" all the way around the base to support the router when it's in the router table, if you look real close at the snapshots you will see I have 3/8" on the one I used, that was a standard PC base plate because I want to use the 1 3/16 brass guides on the table as well,,,,
You could make a base plate to fit your 1.5hp Ryobi that's over size but I don't know of anyone that makes one as a Fac.item.
They can be tricky to make because they need to be dead on, mounting holes and the center hole for the bits...
I'm not sure what size the Ryobi is but you can buy after market base plates from MLCS that are true.
Router Replacement Bases
http://www.mlcswoodworking.com/shop..._sc/store/html/smarthtml/pages/routacc1.html#All_In_One_router_plate_kit_anchor
I also have a Craftsman base plate that is 7" in dia. that will take on the PC brass guides and MilesCraft make a 7 1/2" base plate that will fit most routers BUT it has the TurnLock type guides.., BUT both would work I think on the Ryobi router...

The 7" maybe a just a bit to small but like I said I'm not sure what the Ryobi base is.
Someone will say the plate will spin or turn in place on the router table and that can be fixed easy with a 1/16" roll pin in the router table top and a small slot in the router base plate.
Then one more thing, you must keep in mind is can you drop the router in the router table hole without taking the handles or knobs off the router.
That's why most use the sq. plate..

it gives you room to pop it in the hole easy the norm...the round plate works great for the trim routers because they don't have handles the norm or knobs to hang on to.
The table top I made and the hole I put in is dead on so I didn't need the roll pin to lock it in place or to say keep it from turning.
Drugstore Cowboy said:
Looks GREAT
And raises a brainstorming question for me -
Any reason it would be a less than good idea to use an oversize round base like that to mount a 1.5hp Ryobi --- instead of a large square insert.
My thought is they are both less expensive -- AND --- could be just left on the router when I needed to use it hand held.
Still workin on my first cup of coffee --- so logic may well be flawed ----