Select a thinner base or thicker casing.

Another option, depending on your casing, you can use band molding around your casing that will increase thickness and base will die right into it.

Good luck

You can put a plinth block at the bottom of the casing, a little wider and thicker than the casing. Here's how I did mine:

Is there a 1/8” backer that I can install behind my casing to build it out a little? If so, what product would be used for that? Is there a name for that?

1/8" material you would have to make your own by ripping 1/8" thick strips on the table saw using pine material but you can buy 1/4" lattice molding they come in 1/4 X 1 1/8" or 1 1/2" or 2 1/2" X 8' lengths already primed and it will give you a cleaner edge...

> Is there a 1/8” backer that I can install behind my casing to build it out a little? If so, what product would be used for that? Is there a name for that?

Hahaha,

1) I would follow @ GN Builders L.L.C advice.

2) We were facing the same issue as You. Clearance 2" wood blinds for $10 provided us with abundance 2" x 60" x 1/8" wood slats. [they may have been improperly cut]

BT

I'm curious why you want the baseboard thinner. It will look fine with the same thickness.

As for adding something behind the casing. How do you expect to hide that so it is not visible from the sides?

One option for the casing would be to trim it out with something so the edge of it looks thicker.

Your FLOORING will dictate how much the baseboards must HIDE. If you have solid hardwood (3/4") then you will need a very GENEROUS (at least 1/2" thick) baseboard to hide the expansion gap at the walls. In fact, the rule of thumb for expansion gaps (regardless of how the hardwood/laminate/vinyl is installed) is the gap should be the same width as the product is THICK.

Example: 1/2" floating cork floor = 1/2" expansion gap at all vertical surfaces.

Example: 3/4" hardwood = 3/4" expansion gap at all vertical surfaces.

See how that works?

One of the only ways you can get away with 'thinner' baseboards is if you have finished concrete, sheet vinyl/lino, or carpet. Those are the surfaces that work very well with thin baseboards.

That means the baseboards you are looking at are milled for solid 3/4" hardwood (or any floor that is roughly that thick). That's where these measurements come from...they are there for a reason.

Please look at your flooring to see if there is a correlation between the thickness of the product and the thickness of the baseboard. Now decide whether thinner is better.

Just get some 5/4 stock for casing at a real lumber yard. Rout whatever profile you want.

Or just get 1/2 x 4" baseboard. What is the trim profile you are using?