Get the largest oval speaker and enclosure that will fit into the tender. And don't forget that you may have to drill holes in the tender floor for the sound to escape.
The sound has to be able to escape from the tender, either through holes drilled in the floor or in the top. Removing the weight from the tender floor will indeed make it lighter in weight, but the decoder and speaker combination will add some back.
To determine the size of speaker and enclosure you can squeeze into the tender, you need to open it up, hold the decoder in positiion, and measure the available open space, both with and without the metal weight removed. Most speakers and enclosures are measured in metric sizes, and web sites that sell them give those measurements in mm; i.e., Litchfield Station, Ulrich Models, Tony's Train Exchange, and other web sites have various sizes of round, rectangular, and oval speakers from which to choose.
If the tender has a simulated coal load, you can probably get by with a round speaker mounted under the load and multiple, very small holes drilled through the load using an 80 or so wire-size drill bit. Or if you can do so without it being too obvious, cut a slot or drill a larger hole in the front of the tender and mount the speaker nearby.
And, as David M. Bedard pointed out, a proper enclosure for the speaker is critical to good sound quality and volume.
Here are three types of speakers.
The left one is 20mm X 40mm X 10.26MM, 1 watt from Mouser Electronics. Might go in a Spectrum USRA medium tender.
The middle one is 20.5mm X 27.3mm X 6.23mm, 1 watt from Litchfield.
The one on the right is 1.1mm X 4.37mm, 1 watt from Litchfield.
I have used the round one in my Roundhouse 4-4-0, 2-6-0 and 2-8-0, identical tenders and in my Spectrum 4-4-0.
The small oval in in my Spectrum 4-6-0 firing up through the coal load. I decided against drilling the tender floor.
Rich
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