Normally, if you want to upgrade your receptacles and switch plates it is as simple as going to one of the big box stores and purchasing them. But, if you are trying to match an existing plate, you have to work a bit harder. And if that company / style of plate has been out of production for over 50 yrs, acquiring switch plates is a bit more difficult.
Bryant, in their Uniline Bakelite lineup, never made a Decora style of switch plate. Yes it would have been easier to use a modern plastic brown switch plate, but it is the little details that separate a room designed with big box store reproductions / look-a-likes and one with original features. I obviously chose original features!
Note the 1st image, that is the plate I constructed and the 2nd image is an original with reproduction Kyle Switch Plates 3-gang brown Despard switches. Yes this is 1940's folks!
3rd image is the last and only original switch and plate we had left in the house that we used as the template for the upgrades (downgrades??).

Supplies

Switch plates of your choice with matching screws
A metal switch plate to use as a cutting template
Dremel with a cut off wheel
Drill and bits
1/2" countersink bit
fine tooth rasp, like you would use for laminate counter tops
Paint mask

Step 1: Modify the Decora Inserts

Again it would have been easier to just purchase a Decora insert with a TV coax and a Phone Jack, but everything I found used white push in plates in a universal Decora insert plate. I did not want white squares surrounding the jacks! So I purchased a Phone Jack Decora insert plate and then tried to find just the coax plate adapter. No one had them in stock and they could deliver in 10 days to 2 weeks! Not acceptable! So I purchased the "F" connector as shown and unscrewed the connector and tossed the plastic push in plate.
Then it is just a matter of drilling the hole for the connector, starting on the back side as shown, then finishing up from the front. Then insert the "F" coax connector from the front and tighten the nut from the back. Voila! Semi-instant Decora all brown insert!

Step 2: Step 1 of Switch Plate Modification

The closest Bryant switch plate we could find on Ebay was a double Uniline brown Bakelite toggle switch plate. Close enough!
And after placing a stainless steel double Decora plate on top of the vintage Bryant Uniline plate, all you have to do now is remove everything that is brown!

Step 3: Step 2 of Switch Plate Modification

Note: Definitely wear a face mask for this and a paint spraying filter mask is even better. Bakelite dust is nasty!
Using the Dremel equipped with a cutoff wheel, follow the lines of the stainless steel top plate. When done the 2nd image is what you will see, everything is cut nice and smooth with the exception of the corners.

Step 4: Step 3 of Switch Plate Modification

Here is a close up of the corners. Turn the plate over and cut from the backside until the center pops out.
REMEMBER BAKELITE IS VERY BRITTLE! NO TWISTING OR FORCING!

Step 5: Step 4 of Switch Plate Modification

Even after cutting from the back side, there will still be a bit of Bakelite in the corners. Here is where your fine tooth Rasp comes into play. Carefully remove the corner material until you have a true 90 deg. corner as shown.

Step 6: Step 5 of Switch Plate Modification

Test fit your Decora insert and file away material as needed for a snug fit, but remember to NOT FORCE IT! REMEMBER BAKELITE IS VERY BRITTLE!

Step 7: Step 6 of Switch Plate Modification

With both Decora cutouts complete, mounting holes must be drilled. Again, use your stainless steel "template" and drill the holes. Next, using your 1/2" countersink bit, carefully bevel the holes so the original brown screws fit like factory as shown.

Step 8: Step 7 of Switch Plate Modification

Your switch plate should now be complete, sans wiring. Wiring is 1 antenna coax and 1,4 wire color coded phone line.

Step 9: Finishing Up

Attach our TV to the Coax and our wireless phone box to the phone jack, energizing all the legacy phone jacks in the house and we are done! Now we have created something more in keeping with our 1940's decor than you can purchase at a big box store.